To enhance the measurement accuracy of icing thickness on transmission lines, a novel method for measuring the thickness of uneven icing on transmission lines in complex background images was proposed. Firstly, a series of preprocessing steps, including image grizzling, median filter denoising, and maximum inter-class variance method, were applied to the transmission line icing images. Subsequently, by leveraging the connected domain characteristics of transmission line icing information and background noise, the contour of the icing transmission line was extracted from the complex background. Finally, to address the issue of local uneven icing thickness, a vertical approximation method was employed to obtain the pixel width of icing transmission lines. The experimental results demonstrate that the proposed transmission line target extraction method, based on the parameter features of connected domain, can accurately extract the icing transmission line contour from images with complex backgrounds and low contrast. Moreover, the vertical line approximation method is obviously superior to the traditional Hough transform method in areas with local uneven icing, significantly improving the measurement accuracy of icing thickness on transmission lines.