Abstract: Series of ion thruster product with different diameter,as a kind of magnetic field confining plasma equipments,magnetic circuit has important effect on the ion thruster's performance.In this study,finite element software MAXWELL was used to simulate the magnetic field topography produced by different size of magnet,then the effect of different size of magnet on the thruster's performance was studied.the obtained results indicated that,when the maximum value of the closed magnetic contour was kept 50 G,larger thickness-to-width ratio magnet has produced larger field free region,and this was good to the thruster's beam flatness,thus the thruster's performance could be improved.