In order to make space mirror have superior mechanical and thermal properties, dimensional stability analysis was crucial in the stage of the structural design. Lightweight structure of large aperture space mirror with a 1 200 mm × 450 mm caliber was designed. The mirror blank was made of SiC material. Triangular lightweight holes were laid out on mirror back. The 3 points flexure support scheme was put forward and the mirror support structure was designed. Through analyzing the deformation of the mirror and its supporting structure under gravity load and temperature loads, matching the PV, RMS of the mirror surface shape, and the tilt angle which would be taken as the basis for revising design scheme. As shown as analysis results, under the load case of gravity in X direction, the PV 51.9 nm and RMS 9.2 nm, the tilt angle rotating around Z axis was 2.9 ". Under the action of 4 ℃ uniform temperature rise, the PV was 69.8 nm and RMS was 12.4 nm. The fundamental frequency of mirror subassembly was 173 Hz, which can meet the requirements of space applications.