The on-line monitoring system for the crossing distance of transmission lines is mainly composed of a movement mechanism and a measuring device. At the same time, five transmission lines with clear colors (with the background range can be set according to the field of view) are set at equal intervals (the spacing can be set according to the field of view) within the system's field of view. Large difference) feature mark points. The movement mechanism realizes the adjustment, fixation and recovery of the position of the system device on the transmission line, so as to facilitate inspection and maintenance of the staff. The design of the movement mechanism adopts a simple thread-grabbing movement mechanism similar to the line-following robot, which can be remotely controlled. The fixed installation position of the system is the vertical transmission line directly above the crossover position of the overhead transmission line. Since there are fewer obstacles on the line in the middle section of the tower, the accurate adjustment of the position of the measuring device on the transmission line is realized in a rolling manner. The measuring device realizes online monitoring of the crossing distance of transmission lines at different voltage levels, and is mainly divided into five parts: binocular image acquisition unit, embedded system unit, system communication unit, power supply unit, and remote terminal monitoring center.