In this paper the aberration properties of three-mirror optical system are Investigated. A more directly solving equation method is presented to design this system. The aberration balancing and image evaluation of this system are described. The results show that at the optical system with focal length of 4 meters, relative aperture of f/4, angular field of view 3°, the largest spot radius is 6.35 micrometers and the MTF is 0.36 at spatial frequency 150 linepairs per mm.