Because the water environment capacity of river network in the integrated demonstration area of Yangtze River Delta is limited and the terrain is low, flood disasters usually appear in rainstorm season. Satellite remote sensing technology has become one of the important means of flood disaster monitoring because it is macroscopic and dynamic. In this paper, based on Sentinel-1 C-band synthetic aperture radar (SAR) images, the water body identification method of Dianshan Lake is studied, and the multi-temporal water area extraction is preliminarily realized during plum rain in 2020. Combined with FROM-GLC global land cover type data, the water accumulation situation is analyzed. According to the research results, the following conclusions can be drawn: (1) SNAP software of ESA has a good suppression effect on speckle noise of SAR images; (2) Since the backscattering coefficient of Dianshan Lake water body shows a single-peak shape on the image, the threshold segmentation method can be used to extract water information; (3) The water regime during plum rain increases by about 40 km² compared with that in April. (4) Under the influence of heavy rainfall, the area of submerged farmland is the largest, while impervious surface and bare land are less affected.