The invention discloses a test fixture for in-plane shear test of composite sheets. The test fixture comprises two groups of plugs, two groups of clamping plates and two groups of pressure plates. Each plug is in a T-shaped structure with a cylindrical connector at one end, the far end of each cylindrical connector is provided with a through hole, the near end of each cylindrical connector is provided with a thread used for fixed connection with a fixture port of a tester, and a connection plate is arranged at the other end of each plug and axially perpendicular to the corresponding cylindrical connector. Each clamping plate is fixedly connected with the connection plate of the corresponding plug, and the clamping plates and the pressure plates are provided with through holes in positional correspondence and used for fixing a to-be-tested sample through fasteners. The plugs, the clamping plates and the pressure plates are fixedly connected to two sides of the to-be-tested sample respectively. The invention further discloses a test method adopting the test fixture and a method for manufacturing the to-be-tested sample applicable to the test fixture. The test fixture is capable of realizing shear loading by the aid of the common tester and has advantages of simplicity of a loading device, loading flexibility, shear stress field uniformity and the like.