Abstract: The grids assembly is the key component which influences the ion beam acceleration and extraction of ion thruster，and the thermal deformation displacement caused by heat accumulation during work process，which will influence the performance of ion thruster.FEM（Finite Element Method）is used in this paper to simulate the thermal deformation displacement variation of three-grids assembly with working time.Focusing on distance change is calculated and the temperature balance time is simulated.Warm-up time of the grids and the risk of short circut of the grids with time variation are estimated in this paper.