In order to solve the problem that traditional telescopic systems cannot simultaneously achieve performance requirements such as large field of view and lightweight, the design method of an off-axis three-mirror afocal optical system based on free-form surfaces was researched. Starting from the basic quadratic surface on-axis three-mirror afocal structure, the initial structure of the off-axis three-mirror afocal system was obtained through aperture off-axis processing, and system optimization was carried out on this basis. Introducing Zernike polynomial free-form surfaces to replace traditional quadratic surfaces can further improve imaging quality while reducing system volume and achieving miniaturization requirements. Finally, an off-axis three-mirror telescopic structure with an aperture of 200 mm, a magnification of 5, and a field of view angle of 3° was obtained by optimization. The MTF value in the full field of view of the system is close to the diffraction limit. An analysis of the image quality of the system after the introduction of free-form surfaces was conducted, and the results showed that when only two mirrors were introduced into the free-form surface, the system had already reached the diffraction limit, which can reduce the processing difficulty to a certain extent.