The magnetic non-reciprocity error is one of the main factors that restrict the application of high-precision IFOG, and the error is related to the strength of magnetic field and the twist rate of fiber. The magnetic field sensitivity of fiber coil is more than 10° / h / GS due to the twisting of the fiber, even if permalloy is used to shield the magnetic field, the shielding effectiveness can only reach about 30 dB, which cannot meet the requirements of high-precision IFOG. The influence of the connection gap between shielding materials on shielding effectiveness is analysed by an equivalent circuit model and finite element simulation, the influence of the twist rate on the magnetic field sensitivity is deduced by formula. Through these analyses, the improvements that changed the connection of shielding materials from screw connection to laser welding and made the fiber de-twist are proposed. Through the measure of fiber de-twist, the magnetic field sensitivity of the fiber coil is reduced by 89.3%; through the improvement of laser welding, the shielding effectiveness is improved from 31 dB to at least 64 dB, the magnetic field sensitivity is reduced from 0.0265°/h/Gs to less than 0.0004°/h/Gs, and the bias stability of the IFOG in different temperature is improved by more than 7.5%. These improvements can improve the precision of the fiber coil in the magnetic field and temperature environment.