Aiming at the urgent requirements of military photoelectric system for field of view, image quality and system miniaturization, a large-aperture off-axis three-mirror optical system was designed according to the index requirements. The off-axis multi-mirror structure was adopted to eliminate the system obstruction and reduce the structure size. By introducing the complex profiles, the large-aperture design was realized. While ensuring the sufficient resolution, the field of view angle of the optical system was increased to obtain richer target features and improve the target detection and recognition ability of the system. Compared with transmission-type structure and foldback system, it greatly reduced the volume and weight of photoelectric products and effectively reduced the load of servo design, which had the characteristics of good concealment, wide spectrum coverage, high transmittance and strong field of view compatibility. According to the design results, the full field of view modulation transfer function (MTF) of TV optical path is higher than 0.2 at 120 lp/mm, the MTF of SWIR optical path is higher than 0.3 at 60 lp/mm, the MTF of MWIR optical path is higher than 0.15 at 30 lp/mm, and the grid distortion is less than 0.5%, which meet the design index requirements.