Visible image, compared with SAR image and infrared image, has the advantage of high resolution, clear details, etc. So it can be selected for object extraction. Water objects play an important role in locating bridges, dams and other typical buildings. This paper presents a segmentation method for visible image based on gradient of the original image, and combined with the features of the water targets. According to the feature of water targets, gray uniform, smaller entropy, and smaller local variance, water objects can be extracted automatically and effectively by using clustering method from image segmentation result.