The thermal design of space optical systems is proved by an integrated method of optical-mechanical-thermal analysis. The general method for thermal-optical is explained, and the relationship between thermal-optical analysis and thermal design is expressed. Then, a thermal control system for space cameras is designed by active and pasitive thermal control methods according to its space environment and structure characteristics. The temperature field and thermal elastics at some special boundary conditions are analyzed based on the finite element method, and the displacements and deformations of all the optics surfaces are fitted with Zernike polynomials. Finally, the Modulation Transfor Function(MTF) of space optical system is calculated by CodeⅤ. The results show that the MTF of space optical system is better than 0.5 when its resolution is 50 lp, which meets the requirements of optical design, and proves that the thermal design is reasonable and feasible.