Abstract: Environmental vibrations may cause the positive-branch confocal unstable cavity length to be maladjusted, moreover, thermal effect directly leads to cavity mirrors distortion. As a result, there will be many kinds of phase aberrations in the output beam. In order to correct the defocus aberration, this paper presents two analytic methods to analyze the defocus aberration generated under two different conditions. One is to analyze the relative position misalignment of cavity mirrors leading to defocus aberrations. The other is to analyze the distortion of cavity mirrors leading to defocus aberrations. The relation of the defocus and cavity length as well as the relation of the defocus and cavity mirrors’ distortion are obtained. The conclusion results show that the change of the cavity len