The polarization error of optical system limits the accuracy of fiber-optic gyroscope with double optical length. In order to improve the precision, the models of optical devices and fused points in the new gyroscope are described by using coherent matrix and Jones matrix. Based on the model mentioned above and interference analysis between secondary coupled wave-trains and principal wave-trains in the clockwise and counter clockwise direction, a physical model of polarization bias error of fiber-optic gyroscope with double optical length is established. With practical parameters setting, the influences of some apparatus on polarization noise are simulated and discussed, and furthermore, a new method of matching pigtails is especially presented, which can be used for decreasing the polarization error effectively. Simulation results demonstrate that the polarization bias error can be suppressed from 0.145°/h to less than 0.017°/h, and its peak-valley value of variation with temperature can be suppressed to less than 3×10-4°/h, through matching fiber length. The polarization bias error is suppressed effectively.