Abstract: In the optical system, the apertures of most of target detecting systems on the ground are large (500 mm) which are the on-axis optical systems, and the wide field of view and broad waveband become the most urgent problem to be solved. The catadioptric optical system with small aperture refractive corrector group was designed, which eliminated those aberrations brought by large relative aperture, wide field of view and broad waveband. The primary mirror was aspheric surface and easy to be fabricated. An example which optimized and evaluated with optical design software ZAMAX, and the practical result of observation for star were given finally. The aperture of this practical system was Ф750 mm, whose relative aperture was 1:1.32, the field of view was 4 and the waveband was from 500 nm to 800 nm, the practical capacity of detecting was above 15 Mv. The simple construction consists of several kinds of regular glass, and the imaging quality is very excellent.