Abstract: Single photoelastic modulation (PEM) interferometer has the limitation of thermal and mechanical in the material, so the modulated optical path difference is small, however, series PEMs are difficult to control, and the multiple reflections of various PEM interface make the energy of light lost. Therefore, a method of effectively improving optical path difference by multiple reflections was put forward, based on one single PEM posted on the total reflection film. By deducing and analyzing the influence of the different reflective situation on the optical path difference, the formula of the optical path difference was got after multiple reflections of incidence in arbitrary angle and position. The method was proved by COMSOL, MATLAB, ZEMAX simulations and experiments. The adopted photoelastic crystal was ZeSe and the piezoelectric crystal was quartz. Result shows that the relative error of experiment and theory is 0.21%, which provides the theoretical basis for the engineering application of this method.