Abstract: The design method of a two-sphere system with an afocal corrector which was on the secondary mirror was described. The basic theory for solving the initial structure by using PW method was analyzed. By changing the obstruction ratio K and the length from primary mirror to focus , the ray tracing was realized, the optimized combination of K and was obtained. An design example was given: the working band of the system was 450 nm~700 nm, focal length f was 1 600 mm, relative aperture A was 1/8, visual field angle was 0.5, obstruction ratio K was 0.35, and the length from primary mirror to focus was 0.1. After the optimization of the system, the coefficient of spherical aberration SⅠ-was -0.001 207, the coefficient of comatic aberration SⅡ was 0.000 192, and the wave-front aberration W was 0.043 27.