diff --git a/frontend/src/pages/activity/Detail.jsx b/frontend/src/pages/activity/Detail.jsx
index 848fbf9..ae2f49f 100644
--- a/frontend/src/pages/activity/Detail.jsx
+++ b/frontend/src/pages/activity/Detail.jsx
@@ -23,7 +23,7 @@ const ActivityDetail = () => {
   const navigate = useNavigate();
   const queryClient = useQueryClient();
   const { scrollY } = useScroll();
-  const { login } = useAuth();
+  const { login, user } = useAuth();
   const [loginVisible, setLoginVisible] = useState(false);
   const [signupFormVisible, setSignupFormVisible] = useState(false);
   const [paymentModalVisible, setPaymentModalVisible] = useState(false);
@@ -68,6 +68,28 @@ const ActivityDetail = () => {
     return () => clearTimeout(timer);
   }, [id, refetch]);
 
+  // Auto-fill form fields when the signup form becomes visible
+  useEffect(() => {
+    if (signupFormVisible && user && activity?.signup_form_config) {
+      const initialValues = {};
+      
+      activity.signup_form_config.forEach(field => {
+        // Auto-fill phone number
+        if (field.name === 'phone' && user.phone_number) {
+          initialValues[field.name] = user.phone_number;
+        }
+        // Auto-fill name (nickname) if the field name is 'name'
+        if (field.name === 'name' && user.nickname) {
+          initialValues[field.name] = user.nickname;
+        }
+      });
+
+      if (Object.keys(initialValues).length > 0) {
+        form.setFieldsValue(initialValues);
+      }
+    }
+  }, [signupFormVisible, user, activity, form]);
+
   const signUpMutation = useMutation({
     mutationFn: (values) => signUpActivity(id, { signup_info: values || {} }),
     onSuccess: (data) => {