Abstract: The polarization error of optical system limits the accuracy of fiber-optic gyro. The models of optical devices and fused points in fiber-optic gyro are described using coherent matrix and Jones matrix. The physical model of the optical system in interferometric fiber-optic gyro is established. Compared with the former reports, the optical system described in this model is more engineering oriented. A new method is presented to analyze the polarization error caused by optical devices, fused points and the losses through the optical path. This method could analyze the influence of optical system flaws on bias and scale factor effectively, which can be used to evaluate the property of the optical system in interferometric fiber-optic gyro.