diff --git a/backend-java/src/main/java/com/nanri/aiimage/config/StorageProperties.java b/backend-java/src/main/java/com/nanri/aiimage/config/StorageProperties.java
index 9351f7cf..a371e892 100644
--- a/backend-java/src/main/java/com/nanri/aiimage/config/StorageProperties.java
+++ b/backend-java/src/main/java/com/nanri/aiimage/config/StorageProperties.java
@@ -14,6 +14,8 @@ public class StorageProperties {
     private long sourceRetentionHours = 24;
     private long resultRetentionHours = 24;
     private long transientPayloadRetentionHours = 72;
+    /** 临时目录容量告警阈值（字节），超过时记录告警日志（task-154）。 */
+    private long capacityWarnBytes = 50L * 1024 * 1024 * 1024;
 
     public String getLocalTempDir() {
         String configured = localTempDir == null ? "" : localTempDir.trim();
diff --git a/backend-java/src/main/java/com/nanri/aiimage/modules/file/service/TempDirCapacityMonitor.java b/backend-java/src/main/java/com/nanri/aiimage/modules/file/service/TempDirCapacityMonitor.java
new file mode 100644
index 00000000..4ff2d7b0
--- /dev/null
+++ b/backend-java/src/main/java/com/nanri/aiimage/modules/file/service/TempDirCapacityMonitor.java
@@ -0,0 +1,78 @@
+package com.nanri.aiimage.modules.file.service;
+
+import cn.hutool.core.io.FileUtil;
+import com.nanri.aiimage.config.StorageProperties;
+import lombok.RequiredArgsConstructor;
+import lombok.extern.slf4j.Slf4j;
+import org.springframework.scheduling.annotation.Scheduled;
+import org.springframework.stereotype.Service;
+
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.stream.Stream;
+
+/**
+ * 临时目录磁盘容量告警（task-154）。
+ *
+ * 容量超过阈值时记录告警日志（当前容量/阈值）；告警带频率限制（默认每 10 分钟
+ * 至多一次，防刷屏）；测量失败或目录缺失时仅记日志，不抛异常、不阻塞业务。
+ */
+@Slf4j
+@Service
+@RequiredArgsConstructor
+public class TempDirCapacityMonitor {
+
+    private static final long WARN_MIN_INTERVAL_MILLIS = 10 * 60 * 1000L;
+
+    private final StorageProperties storageProperties;
+
+    private volatile long lastWarnAtMillis = 0L;
+
+    @Scheduled(cron = "${aiimage.storage.capacity-warn-cron:0 */30 * * * *}")
+    public void checkTempDirCapacity() {
+        File tempDir = FileUtil.file(storageProperties.getLocalTempDir());
+        if (!tempDir.exists() || !tempDir.isDirectory()) {
+            return;
+        }
+        long usage = measureUsageBytes(tempDir);
+        if (usage < 0) {
+            log.warn("temp dir capacity measurement failed dir={}", tempDir.getAbsolutePath());
+            return;
+        }
+        long threshold = storageProperties.getCapacityWarnBytes();
+        if (usage > threshold) {
+            warnWithRateLimit(tempDir, usage, threshold);
+        }
+    }
+
+    /** 测量目录总字节数；失败返回 -1（不抛）。 */
+    public long measureUsageBytes(File dir) {
+        if (dir == null || !dir.isDirectory()) {
+            return -1L;
+        }
+        try (Stream<Path> paths = Files.walk(dir.toPath())) {
+            return paths.mapToLong(path -> {
+                try {
+                    return Files.isRegularFile(path) ? Files.size(path) : 0L;
+                } catch (IOException ex) {
+                    return 0L;
+                }
+            }).sum();
+        } catch (IOException ex) {
+            log.warn("temp dir capacity walk failed dir={}", dir.getAbsolutePath(), ex);
+            return -1L;
+        }
+    }
+
+    void warnWithRateLimit(File tempDir, long usage, long threshold) {
+        long now = System.currentTimeMillis();
+        if (now - lastWarnAtMillis < WARN_MIN_INTERVAL_MILLIS) {
+            return;
+        }
+        lastWarnAtMillis = now;
+        log.warn("temp dir capacity warning: usage={} bytes, threshold={} bytes, dir={}",
+                usage, threshold, tempDir.getAbsolutePath());
+    }
+}
diff --git a/backend-java/src/test/java/com/nanri/aiimage/modules/file/service/TempDirCapacityMonitorTest.java b/backend-java/src/test/java/com/nanri/aiimage/modules/file/service/TempDirCapacityMonitorTest.java
new file mode 100644
index 00000000..0e47fa7f
--- /dev/null
+++ b/backend-java/src/test/java/com/nanri/aiimage/modules/file/service/TempDirCapacityMonitorTest.java
@@ -0,0 +1,110 @@
+package com.nanri.aiimage.modules.file.service;
+
+import com.nanri.aiimage.config.StorageProperties;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+import org.springframework.test.util.ReflectionTestUtils;
+
+import java.io.File;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * task-154：临时目录磁盘容量告警契约（plan 09）。
+ * 容量超阈值记录告警（当前容量/阈值）；未超不告警；告警频率受限（防刷屏）；
+ * 测量失败容忍；不抛异常不阻塞业务。
+ */
+class TempDirCapacityMonitorTest {
+
+    @TempDir
+    Path tempDir;
+
+    private StorageProperties properties;
+    private TempDirCapacityMonitor monitor;
+
+    @BeforeEach
+    void setUp() {
+        properties = new StorageProperties();
+        properties.setLocalTempDir(tempDir.toString());
+        monitor = new TempDirCapacityMonitor(properties);
+    }
+
+    @Test
+    void usageMeasuredCorrectly() throws Exception {
+        Files.writeString(tempDir.resolve("a.tmp"), "0123456789", StandardCharsets.UTF_8);
+        Path sub = tempDir.resolve("sub");
+        Files.createDirectories(sub);
+        Files.writeString(sub.resolve("b.tmp"), "12345", StandardCharsets.UTF_8);
+
+        long usage = monitor.measureUsageBytes(tempDir.toFile());
+
+        assertEquals(15L, usage, "容量按字节递归求和");
+    }
+
+    @Test
+    void emptyDirUsageIsZero() {
+        assertEquals(0L, monitor.measureUsageBytes(tempDir.toFile()), "空目录容量为 0");
+    }
+
+    @Test
+    void warningTriggeredOverThreshold() throws Exception {
+        Files.writeString(tempDir.resolve("big.tmp"), "x".repeat(100), StandardCharsets.UTF_8);
+        properties.setCapacityWarnBytes(10);
+        ReflectionTestUtils.setField(monitor, "lastWarnAtMillis", 0L);
+
+        // 超阈值 → 告警路径（频率窗口内可告警）
+        monitor.warnWithRateLimit(tempDir.toFile(), 100L, 10L);
+        long lastWarn = (long) ReflectionTestUtils.getField(monitor, "lastWarnAtMillis");
+        assertTrue(lastWarn > 0L, "告警后记录时间戳（频率限制用）");
+    }
+
+    @Test
+    void underThresholdDoesNotWarn() throws Exception {
+        Files.writeString(tempDir.resolve("small.tmp"), "x", StandardCharsets.UTF_8);
+        properties.setCapacityWarnBytes(10_000);
+
+        monitor.checkTempDirCapacity();
+
+        long lastWarn = (long) ReflectionTestUtils.getField(monitor, "lastWarnAtMillis");
+        assertEquals(0L, lastWarn, "未超阈值不告警");
+    }
+
+    @Test
+    void warningRateLimited() throws Exception {
+        ReflectionTestUtils.setField(monitor, "lastWarnAtMillis", System.currentTimeMillis());
+        long before = (long) ReflectionTestUtils.getField(monitor, "lastWarnAtMillis");
+
+        monitor.warnWithRateLimit(tempDir.toFile(), 100L, 10L);
+
+        assertEquals(before, ReflectionTestUtils.getField(monitor, "lastWarnAtMillis"),
+                "频率窗口内重复告警被抑制");
+    }
+
+    @Test
+    void measureFailureIsGraceful() {
+        File missing = tempDir.resolve("nope").toFile();
+        assertEquals(-1L, monitor.measureUsageBytes(missing), "目录缺失测量返回 -1");
+        assertEquals(-1L, monitor.measureUsageBytes(null));
+    }
+
+    @Test
+    void checkDoesNotThrowOnMissingDir() {
+        properties.setLocalTempDir(tempDir.resolve("absent").toString());
+
+        monitor.checkTempDirCapacity();
+    }
+
+    @Test
+    void warningDoesNotBlockBusiness() {
+        // 告警路径不抛异常（业务不阻塞）
+        ReflectionTestUtils.setField(monitor, "lastWarnAtMillis", 0L);
+        monitor.warnWithRateLimit(tempDir.toFile(), 999L, 1L);
+        monitor.checkTempDirCapacity();
+    }
+}