Y-waveguide to be measured is connected into Sagnac interferometer light path to form complete Sagnac interferometer. Digital stepped wave with period of 4tau, equal step height and step width of tauis applied to the modulation electrode of Y waveguide, the detector output waveform is measured via A/D technology and the difference between level V1 of the 0-tau region and the level V2 of the tau-4tau is used as the criterion. The step height is altered from high to lower, the different between V1 and V2 is calculated in computer, and when the difference is zero the step height is recorded. According to the relationship between the step height and the semi-wave voltage, the semi-wave voltage of the Y waveguide may be obtained correctly. The said method is simple and high in precision and is not affected by light power variation, light path loss and various phase errors.