Compared with the traditional refraction optical system, the off-axis reflection optical system has thecharacteristics of small volume, light weight and no chromatic aberration, and thus it is widely used in the field oftelescope system and space observation system. With the widely use of the free surface, at first, the initial structureparameter determination method of the off-axis three-mirror reflective optical system is analyzed. And then, the freesurface with the Zernike polynomial to design the main mirror of the optical system is applied. At last, the off-axis three-mirror reflective optical system is designed, and its effective focal length is 700 mm, field of view is 10°, rela.tive aperture is F/# is 4.5, total length is 597.58 mm. The modulation transfer function (MTF) in every field angle ishigher than 0.28 at 71.4 lp/mm, and its maximum distortion is less than 0.1%, and the results show the image quali.ty of the optical imaging system is very good. Therefore, the design method of the optical system has the certain ref.erence value for similar off-axis three-mirror optical system design.