Abstract: By using the method of finite element combined with Fox-Li iteration algorithm, the eigenmode in positive-branch confocal unstable resonators has been calculated and simulated, with considering the coupling effect between the eigenmodes of resonators and the thermal distortions of cavity mirrors. The effect of thermal distortions of cavity mirrors on output beam quality of high-power laser has also been analyzed quantitatively. Furthermore, the changes of eigenmode phase characteristics caused by thermal distortions of cavity mirrors have been discussed, and the phase characteristics of output beam with and without thermal distortions have been compared and analyzed from the aspects of the wavefront power spectral density (PSD), Zernike coefficients and factors of output laser beams. The results show that the output beam quality would be influenced by the thermal distortions of cavity mirrors. The proportion of high frequency part in distorted phase and the high order Zernike aberrations increase with the increasing of output laser power, resulting in the degradation of beam quality of output laser beams.