diff --git a/common/i18n/locales/en.yaml b/common/i18n/locales/en.yaml
index 7e7d2ac..dfe4b96 100644
--- a/common/i18n/locales/en.yaml
+++ b/common/i18n/locales/en.yaml
@@ -429,6 +429,10 @@ port_open_http:
   other: "Port open {{.Arg1}} [http](HTTP probe)"
 port_scan_no_alive_subnet:
   other: "Subnet probe found no alive subnets, skipping port scan"
+port_scan_task_dropped:
+  other: "[PortScan] task dropped: {{.Arg1}} ({{.Arg2}}), port may be missed"
+port_scan_tasks_dropped_total:
+  other: "[PortScan] {{.Arg1}} tasks dropped total, these ports may be missed"
 network_rate_limited_pattern:
   other: "Rate limited"
 port_scan_debug_start:
diff --git a/common/i18n/locales/zh.yaml b/common/i18n/locales/zh.yaml
index 9e40f41..035e476 100644
--- a/common/i18n/locales/zh.yaml
+++ b/common/i18n/locales/zh.yaml
@@ -429,6 +429,10 @@ port_open_http:
   other: "端口开放 {{.Arg1}} [http](HTTP探测)"
 port_scan_no_alive_subnet:
   other: "网段预筛未发现存活子网，跳过端口扫描"
+port_scan_task_dropped:
+  other: "[PortScan] 任务被丢弃: {{.Arg1}} ({{.Arg2}})，该端口可能被漏扫"
+port_scan_tasks_dropped_total:
+  other: "[PortScan] 共有 {{.Arg1}} 个任务被丢弃，这些端口可能被漏扫"
 network_rate_limited_pattern:
   other: "发包受限"
 port_scan_debug_start:
diff --git a/core/adaptive_timeout.go b/core/adaptive_timeout.go
index f7d98c5..51ce5bd 100644
--- a/core/adaptive_timeout.go
+++ b/core/adaptive_timeout.go
@@ -25,10 +25,17 @@ type AdaptiveTimeout struct {
 // NewAdaptiveTimeout 创建自适应超时计算器
 // maxTimeout: 用户配置的超时上限（即原始固定超时）
 func NewAdaptiveTimeout(maxTimeout time.Duration) *AdaptiveTimeout {
+	// minTO: 自适应超时下限，取 max(500ms, maxTimeout/5)
+	// 依据：高并发下 TCP 握手存在尾延迟（OS 调度抖动、backlog 溢出、端口竞争），
+	//       过低的下限会导致开放端口被误判为关闭（issue #503）
+	minTO := maxTimeout / 5
+	if minTO < 500*time.Millisecond {
+		minTO = 500 * time.Millisecond
+	}
 	return &AdaptiveTimeout{
 		samples: make([]float64, 64),
 		size:    64,
-		minTO:   100 * time.Millisecond,
+		minTO:   minTO,
 		maxTO:   maxTimeout,
 		warmup:  10,
 	}
diff --git a/core/port_scan.go b/core/port_scan.go
index 9a95820..3a34b6a 100644
--- a/core/port_scan.go
+++ b/core/port_scan.go
@@ -290,6 +290,7 @@ func EnhancedPortScan(ctx context.Context, hosts []string, ports string, timeout
 // slidingWindowSchedule 滑动窗口调度器
 // ants.PoolWithFunc.Invoke 在池满时阻塞，天然提供反压，无需额外 semaphore
 func slidingWindowSchedule(iter *SocketIterator, pool *AdaptivePool, wg *sync.WaitGroup) {
+	var dropped int64
 	for {
 		host, port, ok := iter.Next()
 		if !ok {
@@ -304,11 +305,17 @@ func slidingWindowSchedule(iter *SocketIterator, pool *AdaptivePool, wg *sync.Wa
 		}
 		if err := pool.Invoke(task); err != nil {
 			wg.Done()
+			dropped++
+			common.LogError(i18n.Tr("port_scan_task_dropped", task.addr, err))
 		}
 	}
 
 	// 等待所有任务完成
 	wg.Wait()
+
+	if dropped > 0 {
+		common.LogError(i18n.Tr("port_scan_tasks_dropped_total", dropped))
+	}
 }
 
 // fmtPort 无分配的端口号格式化
@@ -327,7 +334,10 @@ func fmtPort(port int) string {
 	return string(buf[i:])
 }
 
-// connectWithRetry 带重试的TCP连接 - 只对资源耗尽错误重试
+// connectWithRetry 带重试的TCP连接
+//   - 资源耗尽错误：指数退避重试（maxRetries 次）
+//   - 其他错误（如 connection refused、timeout）：直接返回
+//     timeout 是正常的扫描结果（防火墙 drop / filtered），不盲目重试
 func connectWithRetry(ctx context.Context, session *common.ScanSession, addr string, timeout time.Duration, maxRetries int) (net.Conn, error) {
 	var lastErr error
 
@@ -340,9 +350,9 @@ func connectWithRetry(ctx context.Context, session *common.ScanSession, addr str
 
 		lastErr = err
 
-		// 只对资源耗尽类错误重试，端口关闭直接返回
+		// 只对资源耗尽类错误重试，端口关闭或超时直接返回
 		if !isResourceExhaustedError(err) {
-			return nil, err
+			return nil, lastErr
 		}
 
 		// 记录资源耗尽错误
diff --git a/core/real_network_test.go b/core/real_network_test.go
index 74b396b..df95647 100644
--- a/core/real_network_test.go
+++ b/core/real_network_test.go
@@ -440,11 +440,15 @@ func TestReal_AdaptiveTimeout_Convergence(t *testing.T) {
 	if converged >= 3*time.Second {
 		t.Errorf("采样后 Timeout = %v, 应该 < 3s", converged)
 	}
-	if converged < 100*time.Millisecond {
-		t.Logf("Timeout 收敛到 %v（localhost，正常）", converged)
+
+	// minTO 下限断言：max(500ms, 3s/5) = 600ms
+	// localhost RTT 极低，收敛值应贴在地板上（issue #503）
+	minFloor := 600 * time.Millisecond
+	if converged < minFloor {
+		t.Errorf("收敛后 Timeout = %v, 不应低于 minTO 下限 %v", converged, minFloor)
 	}
 
-	t.Logf("AdaptiveTimeout 收敛: 3s -> %v (%d 个样本)", converged, 20)
+	t.Logf("AdaptiveTimeout 收敛: 3s -> %v (%d 个样本), minTO=%v", converged, 20, minFloor)
 }
 
 // =============================================================================