LiNbO3 traveling-wave phase modulator can be used in many fields, such as optical fiber communication, phase-coding quantum key distribution system, and so on. The measurement precision of the half-wave voltage seriously affects the performance of its applied system. A new method is proposed to measure the half-wave voltage based on the Sagnac optical-fiber interferometer according to the phase modulator′s applying model in quantum key distribution system. The data should be reliable compared with other methods because the same modulating pattern is adopted as the quantum key distribution system. This method is demonstrated experimentally with the precision of 10 mV by measuring the half-wave voltage of the phase modulator from Sumitomo corporation, which is used in the differential phase shift quantum key distribution system, and the obtained quantum bit error rate is lower than that of the conventional extreme measure method. This method can be helpful to measure the half-wave voltage of the phase modulator and the loading of phase voltage in the quantum key distribution system.