In order to obtain a wider imaging field of view, freeform surface is used to design a large field of view space optical imaging system. The system uses an off-axis three-mirror optical structure with focal length of 1600 mm, F number of 8, and field of view angle of 20°×1°. Because of the large field of view, the image quality of general aspheric optimization design system cannot meet the requirements. In order to improve the freedom of system design, the Zernike polynomial freeform surface is applied to the tertiary mirror of the system, which enables the sagittal field of view to reach 20° further widening the imaging field of view. Degrees of freedom are increased effectively by the addition of the freeform surface. After optimization design, the optical transfer function of the system is better than 0.5 at 63 lp/mm, and the diffusion spot is optimized into Airy circle. The system energy concentration is high, and the imaging quality is close to the diffraction limitation. © 2019 SPIE.