Taking an optical fiber gyroscope(FOG) circuit board as the example, its magnetic field distribution was analyzed. And by using matrix optics and optical propagation theory, a theoretical model for Faraday nonreciprocal phase error was established in cylinder nonuniform magnetic field. The effects of radiation magnetic field on FOG were analyzed. Results indicate that, the nearer of the distance between the center of magnetic field and the fiber loop, the bigger of the bias error of FOG will be; the relationship between bias and magnetic field direction is a inclined sine, which becomes more gradient when the distance between the center of magnetic field and the fiber loop is nearer; the bias error caused by the nonuniform magnetic field is bigger than that caused by uniform magnetic field of equal intensity, when R＜5r (center of nonuniform magnetic field in fiber loop) or R＜0.5r(center of nonuniform magnetic field out of fiber loop); the existence of nonuniform magnetic field will affect the direction of magnetic axis; and the common circuit board with very weak radiate intensity will also cause unstable output of FOG and directional relativity.