The half-wave voltage was obtained by measuring the phase retardation of the electro-optic crystal under the applied external voltage. Based on polarization interference of birefringent crystal, phase shift of polarized light is transferred to the movement of interference fringes, and the linear accurate measurement is complemented by locating dark fringes position.The experimental results indicate that the accuracy of phase retardation of lithium noibate crystal is 4.4×10－3 rad, the half-wave voltage that LN crystal is measured as 480.0 V with experimental error of 0.10%. Compared with the experimental error of 0.96% that half-wave voltage is measured by extremum method, the measuring error of polarization interferometry method is reduced obviously. The optical structure of polarization interferometry method is simple with high measuring accuracy, the measurement result is insensitive to the power fluctuation of the light source and the measurement range of phase retardation of electro-optic crystal can be largely expanded.