diff --git a/src/Services/SimulationService.cs b/src/Services/SimulationService.cs
new file mode 100644
index 0000000..0e828d9
--- /dev/null
+++ b/src/Services/SimulationService.cs
@@ -0,0 +1,214 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+using GeneralUpdate.Tools.Models;
+
+namespace GeneralUpdate.Tools.Services;
+
+/// <summary>
+/// Orchestrates the full update simulation: server, client, upgrade, log collection.
+/// </summary>
+public class SimulationService
+{
+    private readonly ClientGeneratorService _generator = new();
+    private readonly LocalUpdateServer _server = new();
+    private readonly StringBuilder _fullLog = new();
+    private int _timeoutSeconds = 60;
+
+    public IReadOnlyList<string> LogLines => _fullLog.ToString().Split('\n').ToList();
+
+    public async Task<SimulationResult> RunAsync(
+        SimulateConfigModel config,
+        IProgress<string>? progress = null,
+        CancellationToken ct = default)
+    {
+        var result = new SimulationResult();
+        var sw = Stopwatch.StartNew();
+
+        try
+        {
+            // 1. Validate
+            Log("STEP 1: Validating inputs", progress);
+            Validate(config);
+
+            // 2. Prepare output directory
+            Log($"STEP 2: Preparing {config.OutputDirectory}", progress);
+            Directory.CreateDirectory(config.OutputDirectory);
+
+            // 3. Copy patch to server working dir
+            Log("STEP 3: Setting up local server", progress);
+            var serverPatchDir = Path.Combine(config.OutputDirectory, ".server");
+            Directory.CreateDirectory(serverPatchDir);
+            var patchName = Path.GetFileName(config.PatchFilePath);
+            var patchDest = Path.Combine(serverPatchDir, patchName);
+            File.Copy(config.PatchFilePath, patchDest, true);
+
+            var hash = ComputeQuickHash(patchDest);
+            LocalUpdateServerFiles.Register(patchName, patchDest);
+            _server.Updates.Add((config.CurrentVersion, config.TargetVersion, hash, patchDest, config.AppType));
+
+            await _server.StartAsync(config.ServerPort);
+            Log($"  Server running on {_server.BaseUrl}", progress);
+            config.ServerPort = _server.Port;
+
+            // 4. Generate client/upgrade scripts
+            Log("STEP 4: Generating client.cs and upgrade.cs", progress);
+            await _generator.GenerateAsync(config, config.OutputDirectory);
+            Log($"  client.cs → {config.OutputDirectory}", progress);
+            Log($"  upgrade.cs → {config.OutputDirectory}", progress);
+
+            // 5. Run client
+            Log("STEP 5: Running client (dotnet run client.cs)", progress);
+            var clientResult = await RunDotNetScript(config.OutputDirectory, "client.cs", ct);
+            Log(clientResult.Output, progress);
+
+            if (!clientResult.Success)
+            {
+                Log("  Client failed - see output above", progress);
+                result.Success = false;
+                result.ErrorMessage = "Client exited with error";
+                return result;
+            }
+
+            Log("  Client completed successfully", progress);
+
+            // 6. Verify the patch was applied
+            Log("STEP 6: Verifying update result", progress);
+            await Task.Delay(2000, ct); // Give upgrade process time to complete
+            VerifyUpdateResult(config, result);
+
+            result.Success = true;
+            result.Elapsed = sw.Elapsed;
+            Log($"✅ Simulation complete ({sw.Elapsed.TotalSeconds:F1}s)", progress);
+        }
+        catch (Exception ex)
+        {
+            result.Success = false;
+            result.ErrorMessage = ex.Message;
+            Log($"❌ Simulation failed: {ex.Message}", progress);
+        }
+        finally
+        {
+            try
+            {
+                await _server.DisposeAsync();
+                LocalUpdateServerFiles.Clear();
+            }
+            catch { }
+
+            result.FullLog = _fullLog.ToString();
+        }
+
+        return result;
+    }
+
+    private void Validate(SimulateConfigModel config)
+    {
+        if (!Directory.Exists(config.AppDirectory))
+            throw new DirectoryNotFoundException($"App directory not found: {config.AppDirectory}");
+
+        if (!File.Exists(config.PatchFilePath))
+            throw new FileNotFoundException($"Patch file not found: {config.PatchFilePath}");
+
+        if (string.IsNullOrWhiteSpace(config.OutputDirectory))
+            throw new ArgumentException("Output directory is required");
+
+        // Check dotnet
+        try
+        {
+            var psi = new ProcessStartInfo("dotnet", "--version")
+            {
+                RedirectStandardOutput = true,
+                UseShellExecute = false,
+                CreateNoWindow = true
+            };
+            using var p = Process.Start(psi);
+            p?.WaitForExit(5000);
+            var ver = p?.StandardOutput.ReadToEnd().Trim();
+            if (string.IsNullOrEmpty(ver) || !ver.StartsWith("10.") && !ver.StartsWith("11."))
+                throw new InvalidOperationException(".NET 10.0 SDK is required. Install from https://dotnet.microsoft.com/");
+        }
+        catch (InvalidOperationException) { throw; }
+        catch { throw new InvalidOperationException("dotnet CLI not found. Install .NET 10.0 SDK."); }
+    }
+
+    private async Task<(bool Success, string Output)> RunDotNetScript(string workDir, string script, CancellationToken ct)
+    {
+        var psi = new ProcessStartInfo("dotnet", $"run {script}")
+        {
+            WorkingDirectory = workDir,
+            RedirectStandardOutput = true,
+            RedirectStandardError = true,
+            UseShellExecute = false,
+            CreateNoWindow = true
+        };
+
+        using var p = Process.Start(psi)!;
+        var output = new StringBuilder();
+
+        var readTask = Task.Run(async () =>
+        {
+            while (!p.StandardOutput.EndOfStream)
+                output.AppendLine(await p.StandardOutput.ReadLineAsync(ct));
+        }, ct);
+
+        var errorTask = Task.Run(async () =>
+        {
+            while (!p.StandardError.EndOfStream)
+                output.AppendLine(await p.StandardError.ReadLineAsync(ct));
+        }, ct);
+
+        // Wait with timeout
+        var completed = p.WaitForExit(_timeoutSeconds * 1000);
+        if (!completed)
+        {
+            p.Kill(true);
+            return (false, output + "\n[TIMEOUT] Simulation exceeded time limit");
+        }
+
+        await Task.WhenAll(readTask, errorTask);
+        return (p.ExitCode == 0, output.ToString());
+    }
+
+    private void VerifyUpdateResult(SimulateConfigModel config, SimulationResult result)
+    {
+        // Check if delete_files.json was consumed (it should be gone or applied)
+        var deleteFile = Path.Combine(config.AppDirectory, "delete_files.json");
+        if (File.Exists(deleteFile))
+        {
+            result.Notes.Add("delete_files.json still present - HandleDeleteList may not have run");
+        }
+
+        // Count files changed in app directory
+        var fileCount = Directory.GetFiles(config.AppDirectory, "*", SearchOption.AllDirectories).Length;
+        result.Notes.Add($"Files in app directory after update: {fileCount}");
+    }
+
+    private static string ComputeQuickHash(string filePath)
+    {
+        using var sha = System.Security.Cryptography.SHA256.Create();
+        using var fs = File.OpenRead(filePath);
+        return BitConverter.ToString(sha.ComputeHash(fs)).Replace("-", "").ToLowerInvariant();
+    }
+
+    private void Log(string msg, IProgress<string>? progress)
+    {
+        var line = $"[{DateTime.Now:HH:mm:ss}] {msg}";
+        _fullLog.AppendLine(line);
+        progress?.Report(line);
+    }
+}
+
+public class SimulationResult
+{
+    public bool Success { get; set; }
+    public string? ErrorMessage { get; set; }
+    public TimeSpan Elapsed { get; set; }
+    public string FullLog { get; set; } = "";
+    public List<string> Notes { get; } = new();
+}
diff --git a/src/ViewModels/SimulateViewModel.cs b/src/ViewModels/SimulateViewModel.cs
index 211be8d..600be07 100644
--- a/src/ViewModels/SimulateViewModel.cs
+++ b/src/ViewModels/SimulateViewModel.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.ObjectModel;
+using System.IO;
 using System.Threading.Tasks;
 using CommunityToolkit.Mvvm.ComponentModel;
 using CommunityToolkit.Mvvm.Input;
@@ -11,6 +12,7 @@ namespace GeneralUpdate.Tools.ViewModels;
 public partial class SimulateViewModel : ViewModelBase
 {
     private readonly LocalizationService _loc = LocalizationService.Instance;
+    private readonly SimulationService _sim = new();
 
     public SimulateConfigModel Config { get; } = new();
 
@@ -62,11 +64,44 @@ public SimulateViewModel()
     [RelayCommand]
     async Task StartSimulation()
     {
-        // Validation will be implemented in issue #4
-        if (string.IsNullOrWhiteSpace(Config.AppDirectory)) { Status = "请选择旧版本应用目录"; return; }
-        if (string.IsNullOrWhiteSpace(Config.PatchFilePath)) { Status = "请选择补丁包文件"; return; }
-        if (string.IsNullOrWhiteSpace(Config.OutputDirectory)) { Status = "请选择模拟输出目录"; return; }
-        Status = "模拟功能将在后续 issue 中实现";
+        if (string.IsNullOrWhiteSpace(Config.AppDirectory)) { Status = _loc["Sim.ValidateDirs"]; return; }
+        if (string.IsNullOrWhiteSpace(Config.PatchFilePath)) { Status = _loc["Sim.ValidateDirs"]; return; }
+        if (string.IsNullOrWhiteSpace(Config.OutputDirectory)) { Status = _loc["Sim.ValidateDirs"]; return; }
+
+        IsRunning = true;
+        Log.Clear();
+        Status = "Starting simulation...";
+
+        try
+        {
+            var progress = new Progress<string>(L);
+            var result = await _sim.RunAsync(Config, progress);
+
+            if (result.Success)
+            {
+                Status = $"Simulation completed ({result.Elapsed.TotalSeconds:F1}s)";
+                L($"Result: {(result.Success ? "PASS" : "FAIL")}");
+                foreach (var note in result.Notes)
+                    L($"  Note: {note}");
+
+                // Generate report
+                var reportPath = Path.Combine(Config.OutputDirectory, "simulation_report.md");
+                // report generation will be in next PR
+            }
+            else
+            {
+                Status = $"Simulation failed: {result.ErrorMessage}";
+            }
+        }
+        catch (Exception ex)
+        {
+            Status = $"Error: {ex.Message}";
+            L($"FATAL: {ex}");
+        }
+        finally
+        {
+            IsRunning = false;
+        }
     }
 
     void L(string msg) => Log.Add($"[{DateTime.Now:HH:mm:ss}] {msg}");