A optical system of lander optical payload was designed for the requirement of focusing on objects. A noval focusing type was made that the optical system could keep fixed focal length during focusing. An approaching telecentric optical structure is designed, the optical system was made up of front fixing lens unit and rear focusing lens unit. The front fixing lens is a kind of afocal optical system. The rear focusing lens is at the back of fixing lens unit. The optical can image objects from 0.8 m to infinity in focus by moving rear focusing lens unit. The optical system with spectral range from 400 to 1 000 nm, fixed focal length of 50 mm and F number of 8 can focus on objects located within the range of 0.8 meter to infinity. At last, The modulation transfer function of the optical system can reach to diffraction limit, and distortion is less 1%. The image quality can meet the requirement to the optical system. By the analysis of optical system tolerance, the optical system can keep good imaging quality after fabrication. The optical system can be used to explore moon, mars and asteroids.