Off-axis three-mirror system has the advantages of good imaging quality of vision, large field, compact structure and no central obscuration, so it is increasingly used as the structure of the space telescope. According to the requirement of optical parameters of the optical system design, an off-axis three-mirror telescope system is designed. It has a spectral region from 0.4 μm to 0.8 μm, a focal length of 4000 mm, a relative aperture of 1/10 and a angle of vision of 2.08°. For the designed optical system, the imaging quality is good, at the spatial frequency of 62.5 lp/mm, the modulation transfer function (MTF) value of each field of vision is greater than 0.5, and the imaging quality is close to the diffraction limit. The design result satisfies the requirement. Meanwhile, the effects of mismatch of the primary mirror, the secondary mirror and the tertiary mirror on the MTF of the system, and it is found that mismatch of the secondary mirror shows the greatest influence. This may provide reference for the assemble work of system.