diff --git a/be/src/runtime/cdc_client_mgr.cpp b/be/src/runtime/cdc_client_mgr.cpp
index b37cadc980c920..b60c2c60bb1cbe 100644
--- a/be/src/runtime/cdc_client_mgr.cpp
+++ b/be/src/runtime/cdc_client_mgr.cpp
@@ -214,6 +214,11 @@ Status CdcClientMgr::start_cdc_client(PRequestCdcClientResult* result) {
     argv_storage.emplace_back(java_opts);
     // OOM safety net (last-wins, user opts cannot disable).
     argv_storage.emplace_back("-XX:+ExitOnOutOfMemoryError");
+    // JDK17 opens for debezium ObjectSizeCalculator reflection.
+    argv_storage.emplace_back("--add-opens=java.base/java.lang=ALL-UNNAMED");
+    argv_storage.emplace_back("--add-opens=java.base/java.util=ALL-UNNAMED");
+    argv_storage.emplace_back("--add-opens=java.base/java.math=ALL-UNNAMED");
+    argv_storage.emplace_back("--add-opens=java.base/java.nio=ALL-UNNAMED");
     argv_storage.emplace_back("-jar");
     argv_storage.emplace_back(cdc_jar_path);
     argv_storage.emplace_back(cdc_jar_port);
diff --git a/fs_brokers/cdc_client/pom.xml b/fs_brokers/cdc_client/pom.xml
index a88bddad6830a6..0a0c7355318f7e 100644
--- a/fs_brokers/cdc_client/pom.xml
+++ b/fs_brokers/cdc_client/pom.xml
@@ -76,6 +76,11 @@ under the License.
         <assertj.version>3.27.7</assertj.version>
         <awaitility.version>4.2.1</awaitility.version>
         <maven-failsafe-plugin.version>3.2.5</maven-failsafe-plugin.version>
+        <maven-surefire-plugin.version>3.2.5</maven-surefire-plugin.version>
+        <!-- JDK17 opens for debezium ObjectSizeCalculator reflection (byte-sized queue).
+             Mirrors the set CdcClientMgr passes when BE forks cdc-client.jar; needed because
+             surefire/failsafe run the reader path directly without going through that fork. -->
+        <test.add.opens>--add-opens=java.base/java.lang=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.base/java.math=ALL-UNNAMED --add-opens=java.base/java.nio=ALL-UNNAMED</test.add.opens>
     </properties>
 
     <dependencies>
@@ -258,6 +263,14 @@ under the License.
                     <target>17</target>
                 </configuration>
             </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-surefire-plugin</artifactId>
+                <version>${maven-surefire-plugin.version}</version>
+                <configuration>
+                    <argLine>${test.add.opens}</argLine>
+                </configuration>
+            </plugin>
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-failsafe-plugin</artifactId>
@@ -268,6 +281,7 @@ under the License.
                     </includes>
                     <forkCount>1</forkCount>
                     <reuseForks>true</reuseForks>
+                    <argLine>${test.add.opens}</argLine>
                 </configuration>
                 <executions>
                     <execution>
diff --git a/fs_brokers/cdc_client/src/main/java/org/apache/doris/cdcclient/utils/ConfigUtil.java b/fs_brokers/cdc_client/src/main/java/org/apache/doris/cdcclient/utils/ConfigUtil.java
index a999f532ea9929..cc2fa9da68bbc5 100644
--- a/fs_brokers/cdc_client/src/main/java/org/apache/doris/cdcclient/utils/ConfigUtil.java
+++ b/fs_brokers/cdc_client/src/main/java/org/apache/doris/cdcclient/utils/ConfigUtil.java
@@ -37,6 +37,7 @@
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.google.common.base.Preconditions;
 import com.mysql.cj.conf.ConnectionUrl;
+import io.debezium.config.CommonConnectorConfig;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -123,9 +124,35 @@ public static ZoneId getPostgresServerTimeZoneFromProps(java.util.Properties pro
         return ZoneId.systemDefault();
     }
 
+    public static final String MAX_QUEUE_BYTES_SYS_PROP = "cdc.max.queue.size.in.bytes";
+
+    // Heap-adaptive byte cap for the debezium ChangeEventQueue buffer.
+    // heap 1G->64MB, 2G->128MB, >=4G->256MB. -D<MAX_QUEUE_BYTES_SYS_PROP> overrides
+    // (<=0 disables); a malformed override is logged and ignored, falling back to the cap.
+    private static long resolveMaxQueueSizeInBytes() {
+        String override = System.getProperty(MAX_QUEUE_BYTES_SYS_PROP);
+        if (override != null) {
+            try {
+                long bytes = Long.parseLong(override.trim());
+                return bytes <= 0 ? 0 : bytes;
+            } catch (NumberFormatException e) {
+                LOG.warn(
+                        "Ignoring invalid -D{}={}, expected an integer byte count; "
+                                + "falling back to the adaptive cap",
+                        MAX_QUEUE_BYTES_SYS_PROP,
+                        override);
+            }
+        }
+        long target = Runtime.getRuntime().maxMemory() / 16;
+        return Math.max(64L * 1024 * 1024, Math.min(target, 256L * 1024 * 1024));
+    }
+
     /** Optimized debezium parameters */
     public static Properties getDefaultDebeziumProps() {
         Properties properties = new Properties();
+        properties.put(
+                CommonConnectorConfig.MAX_QUEUE_SIZE_IN_BYTES.name(),
+                String.valueOf(resolveMaxQueueSizeInBytes()));
         return properties;
     }
 
diff --git a/fs_brokers/cdc_client/src/test/java/org/apache/doris/cdcclient/utils/ConfigUtilTest.java b/fs_brokers/cdc_client/src/test/java/org/apache/doris/cdcclient/utils/ConfigUtilTest.java
index 980f83bdc8d4b0..cf70b6b075c77d 100644
--- a/fs_brokers/cdc_client/src/test/java/org/apache/doris/cdcclient/utils/ConfigUtilTest.java
+++ b/fs_brokers/cdc_client/src/test/java/org/apache/doris/cdcclient/utils/ConfigUtilTest.java
@@ -19,11 +19,13 @@
 
 import org.apache.doris.job.cdc.DataSourceConfigKeys;
 
+import io.debezium.config.CommonConnectorConfig;
 import org.apache.flink.cdc.connectors.mysql.source.config.ServerIdRange;
 
 import java.time.ZoneId;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.Properties;
 import java.util.Set;
 
 import static org.junit.jupiter.api.Assertions.assertArrayEquals;
@@ -128,6 +130,63 @@ void tableListEmptyWhenNeitherSet() {
         assertEquals(0, result.length);
     }
 
+    // ─── getDefaultDebeziumProps: queue byte cap ──────────────────────────────
+
+    private static long queueBytes(Properties props) {
+        return Long.parseLong(
+                props.getProperty(CommonConnectorConfig.MAX_QUEUE_SIZE_IN_BYTES.name()));
+    }
+
+    @Test
+    void defaultQueueBytesWithinClamp() {
+        long bytes = queueBytes(ConfigUtil.getDefaultDebeziumProps());
+        assertTrue(bytes >= 64L * 1024 * 1024 && bytes <= 256L * 1024 * 1024,
+                "expected clamp to [64MB, 256MB] but got " + bytes);
+    }
+
+    @Test
+    void sysPropOverridesAdaptiveValue() {
+        String prev = System.getProperty(ConfigUtil.MAX_QUEUE_BYTES_SYS_PROP);
+        try {
+            System.setProperty(ConfigUtil.MAX_QUEUE_BYTES_SYS_PROP, "1048576");
+            assertEquals(1048576L, queueBytes(ConfigUtil.getDefaultDebeziumProps()));
+        } finally {
+            restore(prev);
+        }
+    }
+
+    @Test
+    void negativeSysPropDisablesByteBound() {
+        String prev = System.getProperty(ConfigUtil.MAX_QUEUE_BYTES_SYS_PROP);
+        try {
+            System.setProperty(ConfigUtil.MAX_QUEUE_BYTES_SYS_PROP, "-1");
+            assertEquals(0L, queueBytes(ConfigUtil.getDefaultDebeziumProps()));
+        } finally {
+            restore(prev);
+        }
+    }
+
+    @Test
+    void malformedSysPropFallsBackToClamp() {
+        String prev = System.getProperty(ConfigUtil.MAX_QUEUE_BYTES_SYS_PROP);
+        try {
+            System.setProperty(ConfigUtil.MAX_QUEUE_BYTES_SYS_PROP, "32MB");
+            long bytes = queueBytes(ConfigUtil.getDefaultDebeziumProps());
+            assertTrue(bytes >= 64L * 1024 * 1024 && bytes <= 256L * 1024 * 1024,
+                    "malformed override should fall back to [64MB, 256MB] but got " + bytes);
+        } finally {
+            restore(prev);
+        }
+    }
+
+    private static void restore(String prev) {
+        if (prev == null) {
+            System.clearProperty(ConfigUtil.MAX_QUEUE_BYTES_SYS_PROP);
+        } else {
+            System.setProperty(ConfigUtil.MAX_QUEUE_BYTES_SYS_PROP, prev);
+        }
+    }
+
     // ─── server timezone parsing ──────────────────────────────────────────────
 
     @Test