A new method for outdoor direct measurement of the effect of sun stray light on a camera is presented. When the method is used, the lowest threshold of Point Source Transmittance (PST) can be up to 10-5 which is suitable for the stray light test of large aperture optical systems. The cost of the test equipment is far lower than that of a professional laboratory PST test equipment. The stray light test equipment includes a top opening plant, shutters, adjustable angle tooling, a baffle, a mechanical arm and a camera to be tested. The camera to be tested is placed on the adjustable angle tooling and the shutters are placed on the camera hood. During the test, the sun is shining directly into the camera. According to the response of the detector in the camera, the effect level and distribution of sun stray light on the camera can be evaluated.