From the viewpoint of equivalent sources, the ferromagnetic hull of naval vessels is regarded as a magnetic charge layer, and a Fredholm integral equation of the first kind relating the layer with the measurements of magnetic field sensors is developed. After the discretization of the integral equation, a system of linear equations is obtained which is solved by generalized inverse matrix, and the condition number is employed to assess whether the system is ill-posed. Then the field caused by the hull at an arbitrary point in the space is predicted by the calculated layer. A numerical simulation shows that our method can precisely identify the magnetization to predict the field. And finally a ship model is used to verify the method.