diff --git a/tests/test_ifs_sim.py b/tests/test_ifs_sim.py
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+++ b/tests/test_ifs_sim.py
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+"""
+Identifier:     ifs_sim/tests/test_ifs_sim.py
+Name:           test_ifs_sim.py
+Description:    Test ifs sim.
+Author:         Zhaojun Yan
+Created:        2024-04-09
+Modified-History:
+    2024-04-09, Zhaojun Yan, created
+
+"""
+
+import unittest
+
+from csst_ifs_sim import csst_ifs_sim
+
+
+class TestDemoFunction(unittest.TestCase):
+    def test_ifs_sim_1(self):
+        """
+        Aim
+        ---
+        Test ifs sim function.
+
+        Criteria
+        --------
+        Pass if the demo function returns `1`.
+
+        Details
+        -------
+        The demo function returns the length of the input argument list.
+        This case aims to test whether the demo function returns `1` if input is `None`.
+        """
+        # demo function test
+        dir_path=os.path.join(os.environ['UNIT_TEST_DATA_ROOT'],'ifs_sim/IFS_inputData/configData')
+        
+        csst_ifs_sim(dir_path)
+        
+        self.assertTrue(
+            1 == 1,
+            "Single-argument case passes.",
+        )
+
+    # def test_demo_function_2(self):
+    #     """
+    #     Aim
+    #     ---
+    #     Test demo function.
+
+    #     Criteria
+    #     --------
+    #     Pass if the demo function returns `2`.
+
+    #     Details
+    #     -------
+    #     The demo function returns the length of the input argument list.
+    #     This case aims to test whether the demo function returns `2` if input is `None, None`.
+    #     """
+    #     # demo function test
+    #     self.assertEqual(
+    #         demo_function(None, None),
+    #         2,
+    #         "Double-argument case failed.",
+    #     )
+
+    # def test_demo_function_bad_case(self):
+    #     """
+    #     Aim
+    #     ---
+    #     Test demo function in bad case.
+
+    #     Criteria
+    #     --------
+    #     Pass if the demo function result is not equal to `2`.
+
+    #     Details
+    #     -------
+    #     The demo function returns the length of the input argument list.
+    #     This case aims to test whether the demo function result is not equal to `2`
+    #     if input is `None, None, None`.
+    #     """
+    #     # demo function test
+    #     self.assertNotEqual(
+    #         demo_function(None, None, None),
+    #         2,
+    #         "Triple-argument case failed.",
+    #     )
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