In order to improve the measurement accuracy of null ellipsometry,a null ellipsometry based on photoelastic modulation is presented. Azimuth modulation is achieved through a photoelastic modulator and two quarter-wave plate. The ellipsometric parameters are obtained by azimuth measurement of the polarizer and analyzer,for which the first harmonics of modulator frequency cross the zero. Comparing with traditional null ellipsometry,it can eliminate the effect of background noise and environmental disturbance. The measurement principle is analyzed by using Jones matrix. With measured thickness and optical parameters of ZnS film,the feasibility of the null ellipsometry based on photoelastic modulation is thus demonstrated.