The mode competition coupling effect between two laser modes in double-longitudinal-mode dualfrequency microchip lasers is investigated, and the self-saturation coefficient and cross saturation coefficient of the two laser mode in dual-frequency laser intra-cavity with different refletivities of output mirror are ascertained by experiments. The dual-frequency microchip laser rate equation is established in the research, as the two variables of self-saturation coefficient β and cross saturation coefficient θ are undetermined. Thus the comparison of dualfrequency output power which is obtained by simulation is well illustrated by changing the parameters of microchip laser cavity. By comparing the power envelop profile with dual-frequency laser power spectrum, the two undetermined coefficients are confirmed. The experimental results show that with the refletivities of output mirror at 86%, 81% and 61% respectively, the corresponding self-saturation coefficients are 0.68, 0.66 and 0.52, which means the smaller the reflectivity of output mirror is, the stronger the two mode competition is. A high reflectivity of output mirror indicates the mode competition between the two modes is week, and a large frequency offset of dualfrequency laser can be obtained in such a case.