Magneto-optic Faraday effect is the reciprocal effect that exists in fiber optic gyroscope (FOG). It adds an additional fixed bias which affects the precision in the output of FOG. Based on the characteristic that the radiation magnetic field of internal circuit of FOG is spherical inhomogeneous magnetic field, a magnetic sensibility error model of FOG in the spherical inhomogeneous magnetic field is established. Magnetic sensibility of FOG in the spherical inhomogeneous magnetic field is analyzed by the numerical simulation and verified by the experiment. Shorter the distance between the spherical magnetic field source and optical fiber ring is, larger FOG magnetic sensibility error is. When the spherical magnetic field source is located in the center of optical fiber ring, FOG magnetic sensibility error is the smallest. When the spherical magnetic field source moves around the axial of the optical fiber ring, FOG magnetic sensibility error almost remain unchanged.