Based on the coupling effect of inhomogeneous media on laser transverse modes in resonant cavities, a matrix algorithm for calculating the stable oscillation modes in resonant cavities is proposed. Energy coupling happens between transverse modes during their propagation process in the resonant cavity under the action of inhomogeneous media, and the incoherent transverse modes are coherent gradually. After a few round-trips, the transverse modes are superimposed with each other linearly by a certain energy proportion and form a stable oscillation mode. A coupling matrix of coupling effect for inhomogeneous media is given and the round-trip laser-field is expressed as a vector form. Optical field variations before and after passing through the media are calculated by the matrix calculation algorithm. A stable plane-concave cavity model is built, which is used to study the propagation of the laser field in the resonant cavity with the matrix calculation algorithm. Several stable oscillation modes are obtained under different conditions, which have a good agreement with the ideal modes. The study proves the existence of the coupling effect of inhomogeneous media and provides a new method for fast calculation of stable oscillation modes in resonant cavities.