Models of the Y branch optical modulator′s half-wave voltage with wavelength and temperature are established. The half-wave voltage increases with wavelength increasing and temperature decreasing. A half-wave voltage automatic test system based on Sagnac interferometer modulated with four steps waveform is designed and it can measure half-wave voltage continuously on line. Relative measuring accuracy reaches 8.5×10-6. By changing working wavelength and temperature, the actual measurement of Y branch′s half-wave voltages in 1310 nm and 1550 nm band is realized by using a tunable laser as the light source. The results show that the half-wave voltage is linear with the temperature and the wavelength in the two test bands. The wavelength correlation coefficients are 3.61×10-3 V/nm and 9.54×10-3 V/nm, respectively, and the temperature correlation coefficients are -1.76×10-3 V/℃ and -2.00×10-3 V/℃, respectively. The experimental results verify the correctness of theoretical model.