diff --git a/server/tracker/migrations/0001_initial.py b/server/tracker/migrations/0001_initial.py
index b551169..375fd65 100644
--- a/server/tracker/migrations/0001_initial.py
+++ b/server/tracker/migrations/0001_initial.py
@@ -1,4 +1,4 @@
-# Generated by Django 5.2.17 on 2026-09-15 03:44
+# Generated by Django 5.2.17 on 2026-09-15 03:46
 
 import django.db.models.deletion
 import tracker.models
@@ -65,8 +65,8 @@ class Migration(migrations.Migration):
             fields=[
                 ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                 ('session_id', models.CharField(max_length=64)),
-                ('started_at', models.DateTimeField(blank=True, null=True)),
-                ('ended_at', models.DateTimeField(blank=True, null=True)),
+                ('started_at', models.DateTimeField()),
+                ('ended_at', models.DateTimeField()),
                 ('referrer_domain', models.CharField(blank=True, default='', max_length=200)),
                 ('utm', models.JSONField(blank=True, default=dict)),
                 ('landing_path', models.CharField(blank=True, default='', max_length=500)),
diff --git a/server/tracker/models.py b/server/tracker/models.py
index 84dedf0..ee51b25 100644
--- a/server/tracker/models.py
+++ b/server/tracker/models.py
@@ -56,9 +56,8 @@ class VisitSession(models.Model):
     site = models.ForeignKey(Site, on_delete=models.CASCADE, related_name="sessions")
     visitor = models.ForeignKey(Visitor, on_delete=models.CASCADE, related_name="sessions")
     session_id = models.CharField(max_length=64)
-    # null=True：会话可先建行后回填时间（ended_at 为空表示会话进行中）
-    started_at = models.DateTimeField(null=True, blank=True)
-    ended_at = models.DateTimeField(null=True, blank=True)
+    started_at = models.DateTimeField()
+    ended_at = models.DateTimeField()
     referrer_domain = models.CharField(max_length=200, blank=True, default="")
     utm = models.JSONField(default=dict, blank=True)
     landing_path = models.CharField(max_length=500, blank=True, default="")
diff --git a/server/tracker/tests/test_models.py b/server/tracker/tests/test_models.py
index 601dc81..ecd5777 100644
--- a/server/tracker/tests/test_models.py
+++ b/server/tracker/tests/test_models.py
@@ -18,7 +18,8 @@ def test_visitor_first_touch_fields():
 def test_event_unique_site_event_id():
     site = Site.objects.create(name="s", domains=["a.com"])
     v = Visitor.objects.create(site=site, visitor_id="vid1")
-    s = VisitSession.objects.create(site=site, visitor=v, session_id="sid1")
+    s = VisitSession.objects.create(site=site, visitor=v, session_id="sid1",
+        started_at="2026-09-15T10:00:00Z", ended_at="2026-09-15T10:01:00Z")
     Event.objects.create(site=site, visitor=v, session=s, event_id="e1",
         event_name="page_view", occurred_at="2026-09-15T10:00:00Z", path="/")
     with pytest.raises(Exception):