Output power tuning experimental setup of Y-shaped cavity orthogonal polarized He-Ne laser is built. The mode split frequency difference is changed through changing voltage applied on the end mirror of S sub-cavity (or P sub-cavity), while the common cavity′s length is tuned with the voltage applied on the end mirror of the common cavity varied. It is reported that the light power tuning curves and their beat frequencies variation tune in the cases of different mode split frequency difference with common cavity. Based on the Lamb semiclassic gas laser theory of three-order perturbation, influencing factors and their mechanism are investigated and generalized detailedly. Theoretical analysis shows that longitudinal modes loss, gain per pass and their frequency difference affect the pattern and result of mode competition, which obey the self-consistent equations of light intensity. In the end the light power tuning curves and beat frequencies variation of different frequency difference are theoretically analyzed and explained.