Abstract: Aiming at the problem that the condition number of coefficient matrix is very large after the equation discretization when extrapolating the steel’s magnetic field， a new method was introduced. It can get optimal measurement positions according to magnetic field calculations and mesh of the ferromagnetic object and then the optimal measurement positions can be easily got by particle swarm optimization algorithm. The new method not only optimizes the positions, but also reduces the number of points and receives good results. In order to validate the positions, two different sets of measurement positions were used to extrapolate the sheet steel’s magnetic field. The good results explain that the conclusion is reasonable.