Abstract: The temperature distributions and thermal distortions of Si mirror nonuniformly heated by high energy laser beam are calculated with finite element methods, and the distorted mirror surface is fitted with 65 orders Zernike polynomials for the convenience of ray tracing. The wavefront changes over time with different times of reflection by 45° mirrors are presented, and the corresponding wavefront characteristics such as PV, Strehl ratio and aberration coefficients are tabled. The results show that, distortion occurred on Si mirror nonuniformly heated by the rectangular laser beam with rectangular port, and wavefront aberrations are introduced after multi-reflection. In the aberrations, the y astigmatism and defocus are main contributors. The high order aberration coefficients will gro