Phase retardation of wave plate is measured by analyzing flipping points of polarization states. Measured result and system error are analyzed. Nonlinearity of piezoelectric ceramic is an important error source. The nonlinearity is measured with an interferometer based on microchip Nd:YAG laser feedback phenomenon. The measured system of phase retardation, in this paper, is calibrated to the frequency splitting system. The accuracy of the system is improved. The measurement repeatability is better than 0.12° and the accuracy is better than 0.22°. This paper has a very important significance for wave plate high precision manufacture.