Through the interferencing light intensity under a sine-wave modulation signal analysis, the output signal of fiber-optic gyroscope is demodulated. And then the change of the output signal of gyro is gotten. Through the analysis of the change we know that the effect of residuary intensity modulation on the performance of Interferometric Fiber Optic Gyroscope (IFOG) is related with the precision and accuracy requirements of IFOG. The higher precision, the more obvious effect. Secondly, we analyze the digital ladder height change caused by the demodulation output voltage change in the condition of the effect of residuary intensity modulation on the digital closed-loop fiber-optic gyro. Moreover, through the residuary intensity modulation coefficient and digital ladder height formula, explain the influence of residuary intensity modulation on the fiber-optic gyro zero bias and scale factor. Finally, we simulate the relationship of the residuary intensity modulation coefficient and the scale factor stability of fiber-optic gyro by using the Matlab simulation tool. The simulation results further explain the effect of residuary intensity modulation on digital closed-loop fiber-optic gyro scale factor stability.