In order to resolve the contradiction between high spatial resolution and the tight space of microsatellites, a coaxial ultra-compact primary and tertiary mirror integrated optical system was designed based on the aberration theory and the coaxial four-mirror optical system. The optical system was verified under the condition of a focal length of 1750 mm, an F-number of 7, and a full field of view of 1.4°, and its tolerance and signal-to-noise ratio (SNR) were analyzed. The results show that the overall length of the optical system is less than 200 mm, which is only 1/8.75 of the focal length, suggesting an ultra-compact and small blocking ratio design of the optical system. With increased blocking, the full field transfer function is better than 0.26 and the full field wave aberration is better than λ/50 at a Nyquist frequency of 78 lp/mm. The SNR is better than 24.9 when the altitude of the sun is 30° and the surface reflectivity is 0.05. The system structure is simple and compact, and the imaging quality is good, which has certain reference for the design of high-resolution optical systems for microsatellite.