In the resonator fiber-optic gyro (R-FOG), the detection precision of the system is influenced by the polarization-fluctuation deeply. Using the Jones matrix, the relationship between the drift caused by the polarization-fluctuation and the difference in the coupling ratios of the two eigenstates of polarization (ESOPs) is derived. To make the drift margin, the optimized coupling ratios of the two ESOPs are gotten. These optimized coupling ratios are only related to the loss parameters of the resonator in R-FOG. By the numerical simulation in Matlab, the curves between the drift and the coupling ratios of the two ESOPs are attained, respectively. The polarization-fluctuation error model about the correlation between the coupling ratios of the two ESOPs is set up. Based on the theoretical model above, the drift induced by the polarization-fluctuation is estimated in the reported open-loop experiment of R-FOG. It’s the theoretical base to overcome the polarization-fluctuation and do the experiments in R-FOG system.