According to high-resolution optical system with long focal length of the structure characteristics and structural constraints of the orbital module in diameter 4 m,the length of the optical system is considered one of the important factors for design. Therefore,a long focal length design of the structure above the optical system is particularly important. By the aberration theory and ZEMAX optical design software,the system to meet the ultra-long focal length and high-resolution requirements and the structure of the length is less than 4 meters. A system with a field of view 1.5 °×0.3 °,the relative diameter of 1/9,the focal length 23333 mm length of the optical system and the length of the structure was 3314 mm was designed. Two ellipsoid ,a pair of hyperboloid and two plane mirror to be the folding mirrors are used in the system. The folding mirror can not only shorten the optical path,but also play a role in focusing.