In order to investigate the characteristics of the thermal deformation of the LIPS-300 ion thruster in the working process, a double grid thermal deformation experiment is designed for the ion thruster in the atmosphere, and a loop heater is used to simulate the plasma heat source. It is observed that the spherical grid deforms sharply during the heating process, and the deformation of the screen grid is larger than that of the accelerator grid, which results in the decrease of the grid gap, while during the cooling process, the double grid rapidly recovers and the grid gap increases rapidly. Secondly, the peak drop of the center displacement of the grid in the heating process and the negative displacement in the cooling process are observed. At the same time, it is found that the cooling rate has an important influence on the magnitude of the negative displacement. In order to study the causes of the above phenomenon, the radial displacement of the grid assembly is collected, and the deformation of the grid mounting ring is found to be the fundamental cause. Under the influence of the structure and temperature, the deformation of the grid mounting ring is slower than that of the spherical grid, and its thermal expansion will stretch the spherical grid, resulting in the peak drop of the spherical grid center displacement.