Based on the finite element method (FEM) and Zernike polynomial, the thermal deformation aberrations of Al2O3 mirror in an optical system are calculated. The phase shifts of reflected laser beam and transmitted beacon beam are theoretically analyzed, respectively. The defocusing is the major aberration of reflected laser beam and the defocusing and the primary spherical aberrations are the major aberrations of transmitted beacon beam. The primary spherical aberration is induced by the uneven temperature distribution at the boundary of laser irradiation area. The aberration ratio γnm is defined to describe the influence of absorption power on aberration of reflected laser beam and transmitted beacon beam. The defocusing ratio of reflected laser beam γ20 is 0.0393, and the defocusing ratio of transmitted beacon beam γ20 is -0.0011 and the primary spherical ratio γ40 is -0.0033.