An automatic measuring system based on the principle of non-contact capacitive sensor for inner diameter measurement of the coaxial transmission line outer conductor (diameter:) is introduced in this paper. Through the theoretical analysis, the system also can be used to measure inner diameter of inclined holes. In this system, accuracy positioning is realized in the way that stepping motors cooperate with gratings; accuracy centering is realized by vision CCD system and X-Y accurate slide. Software VC++ is developed for the automatic measuring system. The function of accuracy centering, sensor calibration, automatic inner diameter measurement, data collection, data processing and data analysis are realized by software.