Abstract: In order to meet the needs of space remote sensing optical system in compact structure, small size and high resolution, a design idea and method of compact optical system with long focal length was studied. Based on the Gaussian optics and Seidel aberration theory, the initial structure of the coaxial four-mirror system was built, and the secondary obstructions were avoided by means of field-bias. A large aperture and ultra long focal length coaxial four-mirror optical system was optimized. The aperture of the system was 1 800 mm, the effective focal length was 25 000 mm, and the design full field angle was 1°×0.1°.The design results show that the wavefront of the system is better than 1/50λ(λ=632.8 nm), the relative distortion in the whole field is less than 0.4%, and the total length of this system is only 1/10 of effective focal length (EFL). The structure is simple and compact, the image quality is close to the diffraction limit, and it has certain guiding significance for the design of large aperture and ultra long focal length space remote sensing optical system.