In order to complete the high-precision measurement of the LIPS-300 ion thruster hot gap in the vacuum, high-temperature environment, a non-contact videometrics system using a telemicroscope was designed. Based on the captured image, the interactive partitioning edge detection method was used to obtain stable and clear edges of multiple circular cooperation logos, meanwhile the magnification factor calibration, image distortion correction, horizontal plane correction and hot gap subpixel level deformation detection were performed with cooperation logos. The accuracy verification experiments show that the measurement accuracy of the system is better than 6 μm under non-heating conditions, and the measurement accuracy is better than 12 μm under heating conditions. Experimental results show that the greater the temperature difference between the screen and the accelerator grid, the larger the reduction of the grid hot gap, when the screen grid and the accelerator grid temperature difference is at most 150 ℃, and the hot gap reduction reaches a maximum of 420 μm. At the same time, due to the thermal deformation of the mounting ring, the thermal deformation of the grid decreases during thermal steady state, and the negative displacement occurs during the cooling period. The results of grid thermal deformation measurements are consistent with the trend of similar previous experiments, and the system meets the requirements of ion thruster hot gap measurement.