Abstract: For eliminating the effects of weight and thermal stress on the surface accuracy of a large flat mirror, the surface accuracy was analyzed for the back fixed constraint and the back float constraint structures at -20℃ by means of the finite element method. The design of support structure using finite element method was studied. By using the back fixed constraint and the float constraint at -20℃, the simulation data of mirror surface accuracy was obtained and analyzed comparatively. The back suspension support structure for dynamic scanning mirrors was designed to achieve the back float constraint. After the establishment of detection platform, the profile of the large flat mirror was measured. The results show that the values of peak-to-valley and root mean square of the flat mirror with back suspension support structure were 0.236 and 0.049 respectively. The back suspension support structure could achieve float constraint and release the surface shape deformation of large flat mirrors caused by weight and thermal stress and ensure the surface accuracy effectively.