In order to estimate the overall life of the existing three gate module of 30 cm diameter ion thruster, the grid gap under thermal equilibrium state and the erosion velocity of different regions of the grids were simulated and calculated by FEM (finite element method) and PIC-MCC (particle-in-cell-Monte Carlo collision) method respectively. The results show that when the thruster reaches the thermal equilibrium, the overall deformation of the decelerator grid presents central area depression characteristics, and the overall deformation of the accelerator grid presents a uniform protruding. In the center area of the grids with the diameter of 0~70 mm, the mean gap between the two grids decreases by 0.057 mm. Meanwhile, in the annular region with the diameter of 70~140 mm, the mean gap between the two grids increases by 0.129 mm. The erosion velocity of the apertures at the edge of the decelerator grid reaches 6.25×10^-14 kg/s within 1500 h, and which of the aperture at 5700 h decreases 15.4% compared with 1500 h. The erosion velocity of the apertures at the center and edge of the accelerator grid, and the apertures at the center of the decelerator grid decreases 8.0%，4.1% and 3.6%, respectively. The results of 5700 h life test show that the erosion of the apertures at the center of the decelerator grid, and the apertures at the center and edge of the accelerator grid are basically linear, and the comparison error between the simulation and test is within 10%.