Abstract: The following experiment phenomenon has been reported before:the power varying trend of o light and e light are opposite to each other with the laser cavity tuned,no matter what kind of Ne(Ne20 or the mixture of Ne20 and Ne22) is used and how much the mode split is.Now based on the theory of three level perturbation,the phenomenon is analyzed with the self consistent equations and the mutual gain saturation effect of "the single pair of modes" of the frequency splitting standing wave He-Ne lasers.