Abstract:The halfwave voltage characteristics of Ywaveguide phase modulator used in fiber optical gyroscope(FOG), and their influence on system characteristics of FOG were analyzed. And the effects of temperature and light wavelength on the halfwave voltage were tested. The influence on FOG system of phase drift of Ywaveguide phase modulator were explained. The variations of phase drift with the changes of temperature and relative humidity were studied. Thus the measures for improving Y waveguide technology were put forward according to the experiment data.