On the base of the research object of the optical system of polarization maintaining interferometric fiber optical gyroscope(IFOG), when the light signal is modulation under +pi/2 square wave signal, an input-output model is established by making use of Jones matrix and coherence matrix. Based on the model mentioned above, theoretic analysis on the polarization noise of the optical system is made, and the polarization error expression in the case of the input low polarization light signal is deduced. Simulation analysis the gyroscope output signal and polarization noise error when the laser source performance parameters change. According to the work mentioned above, we proposed that: to design the optical system of IFOG, we should choose appropriate length of pigtail, select the light source, which the extinction ratio of itpsilas output light have high stability to reduce the zero drift to minimum.