To prevent accidents caused by crane arms touching transmission lines, a transmission line anti-external breakage system based on binocular vision is designed. First, by installing the binocular vision anti-external breakage device on the crane arm to obtain image data in real time; then, an improved SGBM stereo matching algorithm is proposed for the light interference, weak texture area existing in the actual environment and edge hole filling in the parallax map, and the algorithm is optimized by using least squares fitting interpolation and bilateral filtering, so as to accurately acquire the three-dimensional information of the transmission line; finally, 10kV transmission line test environment and ranging environment are built according to the principle of binocular visual and space coupling capacitance voltage division and tested in the field. The results show that the average relative error of the improved SGBM algorithm is only 1.345% and 1.229% under the interference environment, which is 2.047% and 1.851% less than the traditional SGBM algorithm and the field strength ranging method, respectively. The ranging system takes into account the needs of real-time and precision, and meets the use of the transmission line crane against external breakage in the actual environment.