Abstract: In this paper, the immersed finite element particle-in-cell Monte Carlo collision (IFE-PIC-MCC) model is used to study the cause of the grid erosion in field emission electric propulsion (FEEP). The simulation results show that charge exchange (CEX) ions are the main cause of the grid erosion, while beam ions basically do not impinge on the grid. The CEX ions are mainly generated near the grid. Some of these CEX ions return to the upper surface, lower surface and notch side surface of the grid. The impact angle of CEX ions returning to the upper and side surfaces is large, but their energy is low, while the impact angle of CEX ions returning to the lower surface is small, but their energy is high. The above characteristics lead to the similar erosion rates of these three surfaces.