Combined with "four position" error problem in inertial navigation system calibration, it was analyzed that the influence mechanism of magnet sensitivity on zero bias stability of FOG in theory, we can obtain the cosine function model between gyro bias and the angle that magnetic field effect on the optical fiber ring; the Influence of magnet field on bias stability is linear, the influence of magnet field with arbitrary directions can be resolved into the sum of two orthotropic magnet fields. Finally, we tested two F120 high precision fiber optic gyroscopes at room temperature and static state, and got zero bias of gyro is 0.2o/ h, 0.3o/ h in all direction range, and zero bias of gyro was decreased to 0.027o/h, 0.038o/h with magnetic shielding, the magnetic shielding can decrease the influence on zero bias of high precision FOG from earth’s magnetic field effectively.