Abstract: The current structural inspection of mine explosion-proof electrical equipment mainly uses the feeler gauge to measure the gap width, which is inefficient and requires equipment to be shutdown. A non-contact explosion-proof electrical equipment plane joint surface gap measurement method is proposed, and an intrinsically safe plane joint surface gap measurement instrument is designed on this basis. The measuring instrument uses a microscope to collect the gap image of the plane joint surface, and obtains the gap width automatically through image binarization, inversion, denoising, cavity filling and other preprocessing methods and through minimum circumscribed rectangle image processing algorithm. The measuring instrument makes the measuring line coincide with the edge of the gap by manually adjusting the position of the measuring line so as to calculate the gap width. According to the actual situation, the gap measurement mode of the plane joint surface of the explosion-proof electrical equipment can be selected by the key of the measuring instrument. The test results show that the gap measurement error of the measuring instrument in the automatic measurement mode is less than 0.02 mm, the measurement process is less than 3 s, and the measurement error in the manual measurement mode is less than 0.01 mm, which meets the requirements for rapid and accurate measurement of the gap of the plane joint surfaces of explosion-proof electrical equipment. When using the measuring instrument to measure the gap of the plane joint surface of mine explosion-proof electrical equipment, the dangerous plane joint surface gap can be quickly determined by the automatic measurement mode, and then the gap can be further measured precisely by using the manual measurement mode.