Abstract: Due to different situations on the ground and in agravic space, special support structure is demanded to unload mirror gravity when testing large-aperture space mirrors on the ground. Meanwhile, deformation induced by clamping forces must be strictly controlled to meet the requirements of high-precision figure testing. This paper presents test experiments performed on a space aspheric mirror made of SiC with 750 mm aperture. Testing results are compared with the finite element analysis results to investigate the influence of the support structure on mirror surface figure. To analyze the causes of incapability of high-precision surface figure testing, the methods of strip suspension and central supporting are discussed. An improved method of backboard supporting is proposed, it reduces the supporting deformation and the repeatability of surface figure at different angles reaches nanometer level. It is proved to be able to achieve higher precision testing, which is a fundamental solution for improving machining quality and efficiency.