Fibe-optic gyroscope is an interferometric sensor, and the characteristic of polarization influences its precision,therefore the polarization state of the fiber need to be tested.At the present,the system of polarization testing is soled abroad,but its index is low,and its price is very high.Accordingly,the testing system used in home is made by ourselves,but its precision is not high universally.At the same time, the tested extinction ratio is smaller than its theoretic value.The design and tested result are given by some companies,yet the reason of the system＇s errors has not analyzed in detail. For this reason, in this article, based on the optical structure of high precision PM fiber polarization detection system,the optical errors that affect the extinction ratio are analyzed and calculated.After theoretically analyzing and calculating by computer simulation,in the range of given errors the extinction ratio can reach 55 dB,which fulfills the high precision measurement.And the transmission ratios of e rays and o rays are obtained by calculation. The transmission ratio of e rays is 2.34% higher than the past results.