Based on Maxwell's equations, under axis-symmetric assumption, a mathematical model of the magnetic field of a Hall thruster is established. A finite difference method is used for discretizing the model. An iterative method for numerically solving the model is given. The numerical results are obtained. From analysis of the results, the effect of the current in the magnetic field coils on the magnetic configuration of a Hall thruster is investigated. Through adjusting the current, the ideal magnetic configurations are found. It was shown that, for ideal magnetic configuration, the magnetic mirror ratio in the inner channel is between 3 and 3.5, and the magnetic mirror ratio in the outer channel is between 0.4 and 0.9. With the increasing of the current in the magnetic field coils, the magnetic field at the exit of the channel increases, but the magnetic mirror ratio cannot be increased. The magnetic field in the inner channel hardly changes with change of the currents in the magnetic field coils.