Bipod structure has the characteristics of static support and can isolate the additional mechanical load. Therefore, it has become one of the common support forms of large aperture space camera mirror assembly. When installing and adjusting on the ground, the surface figure of the mirror supported by the Bipod decreases due to the action of gravity. After the space camera enters the orbit, the surface figure of the mirror will change again with the release of gravity deformation. The gravity error of the mirror assembly is evaluated by finite element analysis method, and its accuracy is difficult to meet the requirements of high-quality and high-resolution imaging. At the same time, the gravity unloading scheme used in the mirror processing process is also difficult to be used to the component stage. In order to solve the problems of aliasing of assembly error and trefoil aberration and insufficient accuracy of spherical aberration test by detection light path in the process of gravity error test, a test scheme combining turnover and unloading was proposed. Based on the orthogonality of different aberrations, individual tests could be carried out to obtain each aberration item by item. Through the gravity turnover test of the mirror, the trefoil aberration in the assembly error and gravity error was separated. The unloading device with certain accuracy was designed. Through the comparative test before and after unloading, the spherical aberration caused by gravity, was obtained. By adopting the above scheme, the measurement of all gravity errors could be realized. The 1.3 m high-lightweight mirror assembly was tested. The gravity error surface figure (rms) and on-orbit surface figure (rms) are 0.192λ(λ=0.6328 μm) and 0.023λ, respectively.