Fiber coil is the sense element of FOG(fiber optical gyroscopes). Except Sagnac effect, magneto-optic Faraday effect is one of the main non-reciprocity effect of FOG. Magneto-optic Faraday effect is caused by fiber twist and residual birefringence during fiber producing and fiber coil winding. It can add one bias in the output of FOG. Based on the finite element analysis,the magnetic model of FOG’s fiber coil is given by using the Jones matrices formalism. The experiment verification is made by using the Helmholtz coil. The result shows that a existing fixed magnet sensitivity axis represents the magnet sensitivity of FOG and the direction of magnet sensitivity axis can be confirmed.And the ways to suppressing the magnetic field effects are discussed.