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

Key words: design of aspheric auxiliary optical system, third order aberration theory, no-aberration double-focus, aspheric testing with semi-transmitting lens;