The axial magnetic sensitivity of fiber-optic depolarized gyro is studied through using Jones Matrix.Based on monochromatic light,the principle of resulting in nonreciprocal phase error that results from axial magnetic is analyzed,and the theory derivation about the actual nonreciprocal phase error is introduced.The emulation result,analysis result and the experiment result are given.And the main method to reduce the Faraday nonreciprocal phase error resulted from the axial magnetic field is put forward,too.The method is that the two depolarizers′ 45° error sum(θ_3+θ_4) should tend to 0.