A dual-ring permanent magnets structure is presented, which can be used as Faraday cell in four-mode differential laser gyro so as to provide appropriate intensity and axial uniform magnetic field. The analytic expression of the magnetic filed distribution along the ring magnet axes is deduced, which is proved thereafter theoretically and experimentally. The conception of magnetic homogeneity is introduced. The distribution curve of the dual-ring magnets is simulated with different intervals and the characteristics of magnetic field distribution are calculated in given homogeneities. In the end, an appropriate design scheme of Faraday cell in four-mode differential laser gyro is proposed.