diff --git a/backend/app/services/alert_service.py b/backend/app/services/alert_service.py
new file mode 100644
index 0000000..a542813
--- /dev/null
+++ b/backend/app/services/alert_service.py
@@ -0,0 +1,101 @@
+"""Alert service: budget monitoring and system alerts."""
+from sqlalchemy.orm import Session
+from typing import List, Optional
+from app.models.alert import Alert
+from app.models.project import Project
+from app.models.worker import AIWorker
+from app.services.cost_service import CostService
+
+
+class AlertService:
+    @staticmethod
+    def check_and_alert(db: Session, tenant_id: int, project_id: int) -> Optional[Alert]:
+        """Check project budget and create alert if needed."""
+        project = db.query(Project).filter(
+            Project.id == project_id, Project.tenant_id == tenant_id
+        ).first()
+        if not project or not project.budget_limit_cents:
+            return None
+
+        spent = CostService.get_project_cost(db, tenant_id, project_id)
+        limit = project.budget_limit_cents
+        pct = (spent / limit) if limit > 0 else 0
+
+        if pct >= 1.0:
+            # Pause project
+            project.status = "paused"
+            db.commit()
+            return AlertService._create(
+                db, tenant_id, "budget_exceeded", "critical",
+                f"项目「{project.name}」预算已超限！已花费 ¥{spent/100:.2f} / ¥{limit/100:.2f}，项目已自动暂停。",
+                project_id=project_id,
+            )
+        elif pct >= 0.8:
+            return AlertService._create(
+                db, tenant_id, "budget_warning", "warning",
+                f"项目「{project.name}」预算使用达 {pct*100:.0f}%（¥{spent/100:.2f} / ¥{limit/100:.2f}）",
+                project_id=project_id,
+            )
+        return None
+
+    @staticmethod
+    def check_worker_budget(db: Session, tenant_id: int, worker_id: int) -> Optional[Alert]:
+        worker = db.query(AIWorker).filter(
+            AIWorker.id == worker_id, AIWorker.tenant_id == tenant_id
+        ).first()
+        if not worker or not worker.max_cost_per_month_cents:
+            return None
+
+        spent = CostService.get_worker_monthly_cost(db, tenant_id, worker_id)
+        limit = worker.max_cost_per_month_cents
+        pct = (spent / limit) if limit > 0 else 0
+
+        if pct >= 1.0:
+            worker.is_active = False
+            db.commit()
+            return AlertService._create(
+                db, tenant_id, "budget_exceeded", "critical",
+                f"Worker「{worker.name}」月度预算已超限！已花费 ¥{spent/100:.2f} / ¥{limit/100:.2f}，Worker已停用。",
+                worker_id=worker_id,
+            )
+        elif pct >= 0.8:
+            return AlertService._create(
+                db, tenant_id, "budget_warning", "warning",
+                f"Worker「{worker.name}」月度预算使用达 {pct*100:.0f}%",
+                worker_id=worker_id,
+            )
+        return None
+
+    @staticmethod
+    def _create(db, tenant_id, alert_type, severity, message, project_id=None, worker_id=None):
+        alert = Alert(
+            tenant_id=tenant_id, alert_type=alert_type, severity=severity,
+            message=message, project_id=project_id, worker_id=worker_id,
+        )
+        db.add(alert)
+        db.commit()
+        db.refresh(alert)
+        return alert
+
+    @staticmethod
+    def list_alerts(db, tenant_id, is_read=None, skip=0, limit=50):
+        q = db.query(Alert).filter(Alert.tenant_id == tenant_id)
+        if is_read is not None:
+            q = q.filter(Alert.is_read == is_read)
+        return q.order_by(Alert.created_at.desc()).offset(skip).limit(limit).all()
+
+    @staticmethod
+    def unread_count(db, tenant_id):
+        return db.query(Alert).filter(
+            Alert.tenant_id == tenant_id, Alert.is_read == False
+        ).count()
+
+    @staticmethod
+    def mark_read(db, tenant_id, alert_id):
+        alert = db.query(Alert).filter(
+            Alert.id == alert_id, Alert.tenant_id == tenant_id
+        ).first()
+        if alert:
+            alert.is_read = True
+            db.commit()
+        return alert