The satellite-Earth laser communication has the characteristics of long communication distance and complex transmission channel. In order to establish a stable and reliable satellite-Earth laser communication link, it is necessary to build a large-aperture ground station. The point position distribution and structure size of the flexible support of the 500mm large aperture primary mirror in the ground station are studied. This allows the primary mirror assembly not only to meet the stiffness requirements, but also to have flexibility to mitigate its own dynamic errors. The optimization process of optical, mechanical and thermal coupling is established on Isight platform.The accurate mirror RMS value is obtained by fitting the surface shape with Zernike polynomial. Taking the RMS value as the optimization objective, the optimal solution of the flexible support point position and the optimal solution of the dimension parameter of the flexible rod are obtained. After the optimization, the temperature range of the primary mirror component is extended by about 30℃ and when the pitch Angle changes, the RMS value of the primary mirror decreases by about 2nm.The main mirror shape meets the requirement of index less than 0.08λ.