The magnetic field sensitivity is the main source of errors in fiber optic gyroscope (FOG) and the optical fiber coil is a major sensitive source. In an external magnetic field, a nonreciprocal phase error will be generated in a FOG and decrease the gyro precision. Related experimental results show that the axial magnetic field sensitivity in the polarization maintaining (PM) fiber coils is more obvious than the radial magnetic field sensitivity. The axial magnetic field sensitivity is studied, including Faraday shift caused by fiber random twist, Faraday nonreciprocal phase caused by geometry twist and non-Faraday nonreciprocal phase caused by vertical component magnetic field. Theoretical results are simulated and verified by the experiments. Results show that nonreciprocal phase caused by vertical component magnetic field is the main reason for axial magnetic field sensitivity and closely related to the fiber coil′s skeleton radius. Furthermore, the smaller the skeleton radius is, the greater the axial magnetic field sensitivity will be.