Abstract: In order to improve modulation performance of Y waveguide modulator so as to improve the overall performance of fiber optical gyroscope(FOG), mechanism for residual intensity modulation (RIM) of Y-waveguide phase modulator used in FOG are analyzed, namely waveguide defects and substrate stray light reflection. Then the influence of RIM on characteristics of FOG system, and calibration factor of RIM influence mainly on FOG are described. Waveform characteristic of RIM of different Y-waveguide phase modulators and its value of linear components and interference are tested. Difference of RIM of different Y waveguide modulators is large. The value of RIM with good performance Y-waveguide modulator can be controlled within 1‰. Influences of temperature on waveform characteristic of RIM are tested. Fluctuations of RIM value can reach 20% under extreme temperature. Finally according to RIM mechanism and ratio of linear interference components, the measures for improving Y waveguide technology are put forward.