The physical models of Si mirror under Gauss laser irradiation was built based on heat transfer theory. The temperature field and thermal deformation distributions on the mirror surface were calculated and analyzed by using Comsol Multiphysics software. And then the effect of thermal distortion of Si mirror on output beam quality of high-power laser has also been analyzed combining with simulation software. The results show that Si mirror thermal deformation increases with the increase of the irradiation time,and beam quality factor β increases accordingly,but β increased significantly and then approaches flat with the increase of the irradiation time. Beam quality and mirror thermal deformation also increase with the increase of the laser power.