Round-way transmission matrix of laser resonator was founded, whose eigenvalue and eigenvector are used for calculating diffraction loss and corresponding mode distribution. The calculated results fit well with the Fox-Li method. The relation of eigenvalue and eigenvector of round-way transmission matrix A2 and one-way transmission matrix A of symmetrical laser resonator, and that of round-way transmission matrix AB and BA of asymmetrical laser resonator, were analyzed. The mathematics expression of transmission matrix relation was founded, which is used for improving calculation efficiency. The calculation discussed the effect of confocal cavity discrete unit amount on the resonator mode. And mathematics expression between optimizing discrete unit amount and radius of resonator mirror was built, and confocal cavity mode with big Fresnel coefficient was calculated.