Abstract: In the field of space optics, the development trend of optical systems is long focal length, large field of view, light weight, large relative aperture, high imaging quality, and so on. In order to adapt to this development trend, the large-caliber reflective optical system is studied.An off-axis three-mirror optical system with large aperture was designed based on the imaging theory of coaxial three-mirror optical system and the way of field off-axis was used to avoid central shielding problem and improve the image quality. The modulation transform function (MTF) of the optical system is more than 0.75 at the Nyquist frequency of 17 lp/mm and the image quality is close to the diffraction limits. In addition, since the tolerance reasonable distribution of optical system is the main factor that affects the performance of camera, the sensitivity analysis and the inverse sensitivity analysis were used to compute the effect of each tolerance parameter on the optical system image quality and the proper tolerance distribution was determined. Through simulation analysis, the optical system can has high image quality with fabricated and assembled on the basis of tolerance given, and the MTF can be more than 0.55.