diff --git a/internal/service/alert_service.go b/internal/service/alert_service.go
index 9e9d5ce..a6e0de2 100644
--- a/internal/service/alert_service.go
+++ b/internal/service/alert_service.go
@@ -1077,16 +1077,11 @@ func (s *AlertService) checkAgentOfflineAlerts(ctx context.Context, now int64) e
 
 		stateKey := fmt.Sprintf("%s:%s:agent_offline:%s", agent.ID, config.ConfigID, agent.ID)
 
-		// 防止时钟回拨导致负数
-		rawOfflineSeconds := int64(0)
-		if now > agent.LastSeenAt {
-			rawOfflineSeconds = (now - agent.LastSeenAt) / 1000
-		}
-		offlineStartTime := config.ConditionStartAfterMaintenance(time.UnixMilli(now), agent.LastSeenAt)
-		offlineSeconds := int64(0)
-		if now > offlineStartTime {
-			offlineSeconds = (now - offlineStartTime) / 1000
-		}
+		// status 由 WebSocket 连接的注册/注销同步维护，是当前是否离线的
+		// 权威状态；last_seen_at 仅用于在已经离线时计算持续时间。在线连接
+		// 的 last_seen_at 会按间隔去抖写库，不能单独据此判定离线，否则
+		// 写库间隔与告警阈值相同时会在边界上反复误报和恢复。
+		isOffline, offlineStartTime, offlineSeconds := agentOfflineCondition(config, &agent, now)
 
 		// 从数据库加载状态
 		state, err := s.AlertStateRepo.GetAlertState(ctx, stateKey)
@@ -1111,15 +1106,12 @@ func (s *AlertService) checkAgentOfflineAlerts(ctx context.Context, now int64) e
 		var shouldFire, shouldResolve bool
 
 		if state.IsFiring {
-			// 已有告警只在探针实际恢复后解除，不能因为维护结束后重新累计而伪造恢复。
-			if rawOfflineSeconds < int64(config.Rules.AgentOfflineDuration) {
-				shouldResolve = true
-			}
-		} else if offlineSeconds >= int64(config.Rules.AgentOfflineDuration) {
-			if !state.IsFiring {
-				shouldFire = true
-				state.IsFiring = true
-			}
+			// 已有告警只在 WebSocket 连接实际恢复后解除，不能因为维护结束后
+			// 重新累计或 last_seen_at 刷新而伪造恢复。
+			shouldResolve = !isOffline
+		} else if isOffline && offlineSeconds >= int64(config.Rules.AgentOfflineDuration) {
+			shouldFire = true
+			state.IsFiring = true
 		}
 
 		// 保存状态到数据库
@@ -1140,6 +1132,22 @@ func (s *AlertService) checkAgentOfflineAlerts(ctx context.Context, now int64) e
 	return nil
 }
 
+// agentOfflineCondition 返回探针当前是否离线，以及扣除维护窗口后的离线
+// 起点和持续秒数。在线状态下时间统一归零，避免去抖写入的 last_seen_at
+// 被误当作连接活性判断。
+func agentOfflineCondition(config *EffectiveAlertConfig, agent *models.Agent, now int64) (bool, int64, int64) {
+	if agent.Status == 1 {
+		return false, 0, 0
+	}
+
+	offlineStartTime := config.ConditionStartAfterMaintenance(time.UnixMilli(now), agent.LastSeenAt)
+	offlineSeconds := int64(0)
+	if now > offlineStartTime {
+		offlineSeconds = (now - offlineStartTime) / 1000
+	}
+	return true, offlineStartTime, offlineSeconds
+}
+
 // fireAgentOfflineAlert 触发探针离线告警
 func (s *AlertService) fireAgentOfflineAlert(ctx context.Context, config *EffectiveAlertConfig, agent *models.Agent, state *models.AlertState, offlineSeconds int64, now int64) {
 	s.logger.Info("触发探针离线告警",
diff --git a/internal/service/alert_service_agent_offline_test.go b/internal/service/alert_service_agent_offline_test.go
new file mode 100644
index 0000000..eef6185
--- /dev/null
+++ b/internal/service/alert_service_agent_offline_test.go
@@ -0,0 +1,43 @@
+package service
+
+import (
+	"testing"
+	"time"
+
+	"github.com/pika-monitor/pika/internal/models"
+)
+
+func TestAgentOfflineConditionUsesConnectionStatus(t *testing.T) {
+	now := time.Date(2026, time.August, 31, 22, 31, 1, 0, time.Local).UnixMilli()
+	staleLastSeen := now - int64(306*time.Second/time.Millisecond)
+	config := &EffectiveAlertConfig{}
+
+	t.Run("在线探针不因去抖后的旧心跳时间误判离线", func(t *testing.T) {
+		agent := &models.Agent{Status: 1, LastSeenAt: staleLastSeen}
+
+		isOffline, startTime, seconds := agentOfflineCondition(config, agent, now)
+		if isOffline || startTime != 0 || seconds != 0 {
+			t.Fatalf(
+				"agentOfflineCondition() = (%v, %d, %d), want (false, 0, 0)",
+				isOffline,
+				startTime,
+				seconds,
+			)
+		}
+	})
+
+	t.Run("离线探针从最后状态更新时间累计", func(t *testing.T) {
+		agent := &models.Agent{Status: 0, LastSeenAt: staleLastSeen}
+
+		isOffline, startTime, seconds := agentOfflineCondition(config, agent, now)
+		if !isOffline || startTime != staleLastSeen || seconds != 306 {
+			t.Fatalf(
+				"agentOfflineCondition() = (%v, %d, %d), want (true, %d, 306)",
+				isOffline,
+				startTime,
+				seconds,
+				staleLastSeen,
+			)
+		}
+	})
+}