The temperature field and thermal deformation distributions of mirror under laser long-time irradiation are calculated and analyzed by using Comsol Multiphysics software. After presenting the wave aberration of mirror with the first 65 order Zernike polynomials, the beam quality factor β at different time is obtained. The results show that beam quality factor β is mainly composed of defocusing aberration, while other aberrations can be neglected. Then an exponential relationship between beam quality factor β and laser irradiation time is offered with the value 0.0052 of room-mean-square error.