Icing on transmission lines has become one of the important factors that endanger the safe and stable operation of transmission lines. The timely identification of ice thickness on transmission lines can effectively prevent the damage caused by ice disasters. In order to improve the measurement accuracy of icing thickness of transmission lines, this paper proposed a linear detection algorithm based on LSD algorithm to detect icing thickness of transmission lines. Firstly, image processing methods such as image pre-processing, morphological processing, and edge detection are used to pre-process the icing image of the transmission line. Then, the edge of the icing image is detected using the LSD algorithm. Finally, the ratio of the image pixels to the actual diameter of the wire is used to find out of ice thickness. The experimental results show that the error of this method is 0.0443, which can be used to measure the thickness of icing.