Abstract: In 2020, a flood disaster occurred throughout Anhui Province due to the persistent heavy rainfall during the super-long plum rain period. To quickly and accurately extract the flood inundation ranges and provide scientific support for flood prevention and disaster relief, this study selected the pre-disaster and mid-disaster Sentinel-1A/SAR data of the Chaohu Lake and Huaihe River basin in Anhui Province. After rapid data preprocessing, this study extracted information about water bodies in the plains and mountainous areas using the Sentinel-1 dual-polarized water index (SDWI) method and topographic factors. Then, it established a monitoring process for flooded areas. Using this process, this study extracted the flood inundation ranges of the Chaohu Lake and Huaihe River basins on July 27, 2020 using the pre-disaster and mid-disaster synthetic aperture Radar (SAR) data. The results are as follows. The SDWI was superior to the backscattering coefficient in the extraction of information about water bodies. The Chaohu basin had a flood inundation area of 524.8 km2 on July 27, and the Baishitian River subbasin was the most severely inundated, followed by the Xihe River subbasin. In the flood flowing and storage areas of the Huaihe River basin within Anhui Province, the flood inundation area of four cities along the Huaihe River basin decreased in the order of Huainan City, Fuyang City, Lu’an City, and Bengbu City. The results of this study show that the Sentinel-1A-based monitoring process of flood inundation areas established using SDWI and topographic factors has high accuracy, applicability, and timeliness for plains and mountainous areas and is convenient for the timely monitoring of flood disasters in these areas.