In the space, temperature is the key factor influencing the optical system imaging quality. To investigate the effect of temperature on the off-axis three mirror system,a thermal optical analysis method from finite element analysis to optical analysis was presented.On the basis of analysis results of the optical system in uniformly distributed temperature fileds,the surface aberration of mirrors with temperature gradients was obtained by using a finite element analysis software, and the surfaces with aberration were fitted with the Zernike Polynomials. Then,the Zernike coefficients were carried into the software CODE V, the imaging qualities of the system with difference temperature gradients were analyzed, and thermal control requirements were determined. Finally,a thermal optical experiment was carried out to test the optical system with active thermal controls. Experimental results show that the MTF of full field in the optical system is more than 0.2 in 56 lp/mm,which means the optical parameters meet the the requirements of optical system.Results also verify the correctness of the theory and the reasonabity of thermal control requirements.