The output characteristics of polarization maintaining fiber optic gyro were studied in the influence of the axial magnetic field parallel to the sensitive axis of the sensing coil. The mathematical model of the relationships between the nonreciprocal phase difference in polarization maintaining fiber optic gyro and the polarization state of incident light was established and verified by experiments. The results show that the drift caused by the magnetic field in the polarization maintaining fiber optic gyroscope come from the fiber’s bending when coil is wounded, and the drift is closely related with the distribution of the fiber in the coil. The sensibility will not change after the sensing coil is completely manufactured because of the invariant distribution of the fiber twist. By changing polarization state of the incident light to some extent, the sensitivity of the axial magnetic field of polarization maintaining fiber optic gyro can be changed.