In the optical system of space camera, the optical window is the only optical component exposed to the very complicated thermal vacuum circumstance in orbit space. The temperature gradient occurs imperatively and then influences to the image quality and resolution of the space camera. Based on the theory of optical wavefront error and combining the engineering practice, the thermal optical evaluation has been done to the optical window with non symmetric temperature field. The RMS wavefront errors of the optical window have been calculated on each typical position in orbit cycle. The results of the thermal optical evaluation in this paper are verified by the optical windows of American “Skylab” spaceship as a reference and a proof.