The invention provides an image detection method and system for composite insulators based on a Lab color mode. The method comprises the steps of acquiring of a dry composite insulator surface image and water spray treatment of the surface of a composite insulator. The method also comprises the steps of acquiring an image of the surface of the composite insulator after water spraying; adjusting the color mode of the acquired image; extracting the difference of color space of the image; building a gray-scale map of water drop information; extracting a water drop area by using the maximum gray scale entropy. Through comparison of two images and conversion of Lab color space, water drop distribution at the surface of a composite insulator can be judged accurately, the misjudgment rate and the judgment missing rate are reduced, the anti-jamming capability of image detection is improved and water drop information of the surface of a composite insulator can be extracted effectively.