The primary mirror system is the key component of the high-precision optical system, and the surface accuracy of the primary mirror determines the imaging quality of the whole system. When the lightweight mirror is affected by its own weight, support mode, load and temperature, the surface shape accuracy of the primary mirror will be changed, and the assembly mode of the primary mirror is an important factor to ensure the surface shape accuracy of the primary mirror. There are usually three types of assembly modes for the primary mirror: peripheral support, central support and back support. In this paper, the supporting mode of 650mm lightweight mirror is studied, and the surface shape of the mirror is analyzed under two supporting modes (center support and back support) when the elevation of the mirror is between 0 ° and 60 °, which is affected by its own weight. Firstly, the primary mirror is modeled based on CAD; then, the CAE mesh is generated according to the CAD model with different support modes, and the structural deformation of the primary mirror under different working conditions is obtained by solving the structure of the primary mirror under different working conditions; finally, the mesh data of CAE analysis is extracted, and the mesh data is subjected to polynomial fitting to obtain the deformation of the secondary mirror in different working conditions and States. Based on the method of simulation analysis, this paper adopts a variety of simulation tools, including CAD, CAE, multidisciplinary fitting analysis software and so on. The deformation of the primary mirror under different working conditions and different support modes is obtained. When the 650 mm lightweight mirror is supported on the back, the surface shape change does not exceed RMS 0. 01 λ at different elevation angles. For the center support, the surface shape change is RMS 0. 004 λ to RMS 0. 12 λ in the elevation angle range of 0 ° to 60 °. This supporting mode has a great influence on the surface shape. Based on the simulation analysis data, the back support is used for the assembly of the 650 mm lightweight mirror, and the center support is not suitable for the assembly of the mirror. © COPYRIGHT SPIE.