Interferometric fiber optic gyroscope (IFOG) is affected by various physical fields such as particle radiation and electromagnetic field in space environment, which deteriorates the angular velocity output error. In this paper, the magnetic field error model of fiber optic gyroscope was derived by analyzing the main sources of the magnetic field error of fiber optic gyroscope. Based on the magnetic field error model, the influence of radiation on the parameters of magnetic field error was investigated and the theoretical model of radiation-induced magnetic field error was established. Based on the model, by the influence mechanism analysis and experimental verification, it is determined that the radiation mainly affects the output zero bias of IFOG by changing the Verdet constant and stress-induced birefringence. Furthermore, by establishing a miniaturized IFOG prototype, the polarization maintaining fiber coil was radiated and the corresponding magnetic field error was measured. The experimental results show that the magnetic field error of IFOG varies with the total radiation dose, and its variation law conforms to the radiation-induced magnetic field error variation model.