Abstract: The plasma particle simulation of ion thruster optical system was carried out by the PIC (particle-in-cell) method. PIC method is an efficient method in plasma particle simulation, in which the SOR(successive over relaxation) method was adopted to calculate the electric field and the leap-frog method was adopted to accelerate the particles. Xenon was used as propellant and only singly charged Xenon ions was included in the simulation. The simulation results include the distribution of the electric potential between the grids and distribution of the electric field strength, and also the distribution of Xenon ion number density. The simulation results show that under the electrical parameters and the geometric parameters used, the ion beam can get across the grid aperture, with no hits on the grid wall. The simulation of the optical system provided an efficient method to analyze the characteristic of the grid erosion and to evaluate the life time of an ion thruster.