The semi-transmitting lens is used in the test of aspheric surface. The semi-transmitting surface and the surface under testing are close to each other in space as two surfaces of the lens. Retro-reflected surface can also compensate the aberrations. This testing can eliminate the obscuration, decrease the size of optical system and increase the ability of compensation. According to the third order aberration theory, a method to obtain the testing parameters of the surface under testing is given. An optical system to test a convex aspheric mirror with diameter of 300 mm and focus of 1200 mm is designed. After the optimization with ZEMAX, the wave front aberration of the testing system is lower than 1/(100 λ), which meets the requirement of optical testing.