Abstract: The thermal distortion effect of resonator mirror in high power gas lasers was investigated. In terms of the method of time discreteness, a 3D physical optic loaded model for folded confocal unstable resonators in negative branch and ANSYS models for thermal effects was used to couple the fast-varying cavity modes and slow-varying physics to mirror distortion. We sloved the interaction between the thermal distortion of mirror and cavity modes. It was shown that, in 16 s running, the change of beam quality was less 1.5 times of diffraction limit for Si mirror, while that is 4 for SiO2 mirror. It is concluded that Si mirror works well than the SiO2 mirror in this kind of designs.