By using interference method of polarization,the relation of input voltage and phase delay of phase-polarization modulator is discussed in detail,and the expression of the intensity of output interference light and phase delay of phase modulator is deduced. The result shows that the phase-polarization modulator can obtain steady interference light by using interference method of polarization,so the voltage-phase characteristic curve of phase-polarization modulator can be measured accurately. The result of the measurement shows that voltage-phase characteristic curve of phase-polarization modulator has good linearity.