Abstract: The coaxial eccentric-pupil system has the advantages of long focal length, wide field of view and small volume. It overcomes the shortages of the limited field angle of the coaxial system, and the difficult of manufacture and assembly of the off-axis reflective system. It meets the requirement of high-resolution space-to-earth observation. The design process of coaxial eccentric-pupil three-mirror system is introduced. The coaxial eccentricpupil three-mirror system with focal length of 3000mm and F number of 10 was designed. The result indicates that the field of view of the system can attain to 80.8, and MTF is higher than 0.55 at 50lp/mm. The image quality of the optical system approaches the diffraction limit, and can satisfy the requirement of the system.