In order to obtain ideal circularly polarized laser in the four-mode differential laser gyro, the influence of cavity parameters on polarization degree is analyzed with perturbation method. The perturbation matrix to the first order approximation is derived. Not only anisotropies of cavity mirrors but also optical activity and linear birefringence of quartz crystal are considered for planar four-mode differential laser gyro, while anisotropies and nonplanar rotation angle of carity mirrors are considered for nonplanar four-mode differential laser gyro. The first order perturbation solution of eigen-vector is obtained with perturbation method for eigen problem of matrix. The variation of polarization degree as a function of anisotropies of cavity mirrors is discussed when the rotation angle of the cavity is 90°. Expressions derived from perturbation method agree well with strict numerical solutions when polarization degree is less than 0.05. Polarization degrees of four-mode differential laser gyros as a function of anisotropies of mirrors are different between palanar and nonplanar cavity. The results have certain instructive effect for optimizing the cavity design of four-mode differential laser gyro.