The theory that the magnetic field vertical to the sensing coil plane may induce nonreciprocal phase difference (NPD) in polarization maintaining fiber (PMF) optical gyroscope is put forward. The corresponding mathematical model is established. Simulated analysis and experimental verification are carried out. The magnetic field error induced by vertical magnetic field in polarization maintaining fiber optical gyroscope arises from the bending of the fiber. And the error is closely related with the diameter of the optical fiber coil, fiber′s diameter, fiber′s length and the strength of the vertical magnetic field and so on. The simulation analysis and experimental result show that, as to a fiber optical gyroscope, in which the angles of the two tail fibers between the integrated optical components (IOC) and the fiber loop are 0°, and the magnetic drift is random in some range. But in which the ones are 45°, the magnetic drift is very stable and is linear to the magnitude of the vertical magnetic field.