The drift of an interferometer fiber-optic gyro(IFOG) by the magnetic field through the Faraday affect is systemically demonstrated.Based on the Jones matrix,a generally formula that can be used to analyze this problem quantitatively is given.In the analysis,a model in which several polarization maintaining optical fibers are connected in series is used to describe the characteristics of the ordinary single-mode fiber.By the derived formula,the drift caused by the magnetic field in the IFOG with a polarization maintaining fiber is simulated.The simulation result is given.The precision requirement of the fiber rotation system used to splice the fiber coil with the gyro chip is gained from the result.