The root-mean-square (RMS) phase distortion and the strehl ratio in the far-field influenced by the thermal distortion of mirrors in the propagation of high power lasers were calculated. Calculations with different CO2 laser beam intensity distribution indicate that compare with stable cavity whose output is Gaussian, unstable cavity’s beam quality influenced by the thermal deformation depends on the slope of the intensity distribution, that is, contents of H2O, and the obscuration. Among the mirror materials such as SiO2, BeO, CaF2 and Si, CaF2 provides the steadiest thermal character in the high power laser irradiance. When the SiO2 mirror is cooled, the RMS phase aberration brought by the thermal deformation would be less. For a high power mirror, cooled by the water is more efficacious than cooled by the wind.