The magnetic field in fiber optic gyroscope (FOG) will cause Faraday effect, resulting in Faraday phase error. Experimental results show that the axial magnetic field sensitivity is more obvious than the radial magnetic field sensitivity. The clockwise (CW) and coienter- clockwise (CCW) light which propagate in the polarization maintain (PM) fiber in Fiber- optic gyro will generate a nonreciprocal phase error associated with axial magnetic field. In this paper, the geometrical axial magnetic field sensitivity caused by helically wounded optical fiber is studied; coupled- mode theory and finite element analysis are used to induce the FOG′s specific expression of Faraday nonreciprocal phase difference generated in axial magnetic field. The simulation analysis is given based on the theoretical results. Research show that circular birefringence caused by the geometrical torsion in the fiber of the fiber coil is the main causes of the geometry Faraday phase error. Furthermore, the study results show that axial magnetic field sensitivity will increase with decreasing radius.