The Forward Passage Gain (FPG) is an important factor that affects the detection Signal-to-noise Ratio (SNR) and dead band nonlinearity of Fiber Optic Gyroscope (FOG). If FPG's value becomes lower, the SNR and dead band nonlinearity will be worse. The mathematical formula of FPG is deduced from the physical model, and the influence of space environment on the FPG is analyzed. The pseudo-random identification method is used to on-line estimate the FPG of the FOG. The pseudo-random signal is put on the sawtooth as the identification input signal. The pseudo-random demodulation works to extract the FPG. At the same time, the square-wave demodulation works to extract FOG's error signal. The correlation among modulated error signal, square-wave demodulation signal, pseudo-random disturbance and the pseudo-random demodulation signal is calculated, and the conclusion is made that pseudo-random demodulation will not bother the working of the FOG's demodulation process. The hardware realization is introduced and during the radiation test of the SFS components, the pseudo-random identification is validated. The test result shows that FPG can reveal the changing of output power from the SFS and the Random Walk Coefficient (RWC) of FOG.