The off-axis reflection system has the characteristics of no chromatic aberration, no central obscuration and compact structure, but due to its asymmetric structure, its field of view is often a linear field of view. An off-axis three-mirror system with a large symmetric field of view was designed based on Zemax. The field of view is 13°×13°, the relative aperture is 1/5, the focal length is 800 mm, and the wavelength is 400 mm~1 000 mm. The primary mirror and the third mirror are free-form surfaces, and the secondary mirror is a quadratic surface. The design results show that the root mean square of spot diagram diameter of the proposed system is less than 5 μm, 80% of the energy is in the one pixel, the optical transfer function is close to the diffraction limit, and all the indicators meet the application requirements.