Image edge detection technology which is applied to the hydrophobic grade discrimination of composite insulator is of great significance. How to accurately detect the edge of the water droplets is very important in the process of automatic identification of composite insulator hydrophobicity. This paper detects composite insulator hydrophobic image edge with image edge detection technology. Firstly, adaptive histogram equalization technique is used to enhance image, reducing the similarity of target and background which is caused by the transparency of water. Then the paper uses the improved Canny algorithm which is introduced Ostu threshold method to detect the image edge. The experimental results show that the improved algorithm can accurately acquire the water droplets edge of composite insulator surface, which can lay a foundation for automatic discrimination of composite insulator hydrophobic grade.