The test result of space mirror figure in the ground environment is mainly composed of the surface processing residual and the distortion caused by gravity and support. In order to realize the gravity-support distortion separation of the 1.2 m lightweight space reflector, by testing mirror with equal gradient support forces, the relationship between mirror figure distortion and the change of support forces is obtained from the mirror mechanical response, which can be used as a basis for judging the result of finite element simulation and optimizing finite element model. After removing the mirror figure distortion calculated with the corrected model from the mirror test result, the surface processing residual is obtained. The research shows that the differece of mirror distortion change caused by 100 N supporting force change between the result calculated with the corrected model and the test result is ≤0.001λ, without the effects of gravity and support, the surface processing residual of space mirror with test result rms 1/30λ is better than 1/40λ. This result can not only guide the high precision surface polishing, but also improving the alignment precision of final system.