Abstract: In view of the difficulty of extracting water information of small water body area, this paper puts forward a new method, by which Sentinel-1A SAR is used as the data source, and the simplified superpixel segmentation algorithm SLIC is used to segment the image, the segmented image is merged according to the similarity and binarized to obtain the final water body information. Compared with the water information extracted by Otsu threshold method, the proposed water extraction algorithm effectively improves the efficiency and accuracy of water extraction, and has important scientific value for flood control and disaster reduction. The method is applied in the Zhuzhou flood monitoring in July 2019 and achieves good results.