In order to get reflection surface deformation after assembly of support system, the finite element method (FEM) is used to analyze the primary reflection mirror structure. Surface shape and self-weight deformation of a corbeling system at different tilted angles are analyzed, and deformation displacement on evey pitching height of mirror surface and its corbeling system are obtained. The anlaytical results show at different tilted angles, displacement in Z direction is more than that in X and Y direction, and its maximal displacement is 1.4 μm that translates the reflection surface and changes the focal distance. The deformation of X (slewing axis)direction is 0.01″. And the Y direction is the mirror tilting direction, and its deformation is about 0.5″ and bends the mirror surface.