Freeform surface is applied to design a large field of view off-axis three-mirror optical system. The focal length of the system is 2 000 mm, the F-number is 12, and the field angle is 35°×1°. In the system, XY polynomial freeform surfaces are applied as the primary mirror and the tertiary mirror surfaces, and the primary mirror is designed as a convex surface, so that the meridional field of view reaches 35°, further widening the imaging field of view. Based on the optimized XY polynomial coefficients, Matlab software is used to simulate the XY freeform surfaces of the primary mirror and the tertiary mirrors. The design results show that the optical transfer function of the system in the full field of view is better than 0.4 at 63 lp/mm, the diameter of the diffusion spot is less than one pixel size, the maximum relative distortion is less than 3%, and the wavefront error value is better than λ/14. The system has high energy concentration and the imaging quality is close to the diffraction limit. It can be seen that the freeform surface has great advantages in improving the imaging field of view and imaging quality of the off-axis reflection optical system. The system overcomes the shortcoming of the small field of view in the radial direction of the conventional off-axis reflective optical system, and it is suitable for wide swath imaging.