Abstract: Based on Immersed Finite Element Particle-In-Cell（IFE-PIC） and Monte Carlo collision（MCC） method,a three dimensional numerical model is adopted to investigate the plasma flow and the kinetic characteristics of the charge exchange（CEX） ions of ion thruster.The simulation results reveal the ion density distribution and impingement current distribution of erosion depth of the downstream surface and aperture on the accelerator grid of the optical system.It is indicated from the simulation results that the CEX ions originated in the downstream region of the optics are the main reason of the accelerator grid current and the erosion on the downstream face of the accelerator grid.The erosion pattern agrees with the"pits and grooves"erosion pattern.The numerical simulation provides an efficient tool to design ion thruster optical system.