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It is significant to obtain the flood inundation area accurately in time in terms of fighting against flood disasters. But it is often rainy and cloudy when flood disaster happens, so the optical remote sensing is not able to survey the flood inundation area, which will decrease the timeliness and accuracy. The Synthetic Aperture Radar (SAR) can be applied to determine the flood inundation area and flood regions of the underlying surface in time, for it is not influenced by clouds and rains and can offer allweather observation. Therefore, SAR can provide a strong guarantee for flood survey. In this article, the flood disaster in the Yangtze River region occurred in July 2016 is used as an example, in which the Sentinel1 SAR data, along with threshold and RGBband methods, are used, by means of GIS tools, to successfully extract the water areas from some lakes of the Yangtze River as well as the main stream of the Yangtze River. As a result, the submerged range and the flood area of each county are obtained. The flood area is analyzed and compared with the results of the MODIS monitoring in the same period. The results show that the method of water extraction and flood area calculation based on Sentinel1 is effective, which can be the basis of researches on the application of Sentinel1 data in flood disaster evaluation.