With the development of space technology, new design requirements for optical systems of space cameras are provided. Higher accuracy requirements are provided for system processing so as to improve the reli. ability of camera systems. An easily fabricated optical system of space camera is introduced. A catadioptric opti. cal system with three pieces of spherical collimating mirror subassembly based on ritchey chretion (R-C) system is designed. It shows that when the space frequency of the system is 50 lp/mm, all the modulation transfer func. tions are more than 0.4. And the surface of primary and secondary mirror are quadric, the rest is spherical, which has the advantages of easiness in fabrication, installation and debugging.