Abstract: In the flood season of 2020, several heavy rainfalls occurred in southern China, which resulted in severe floods in many places in the middle and lower reaches of the Yangtze River. Spatial information technology which based on GIS and RS has a strong spatial analysis and monitoring ability in finding, positioning and analyzing flood risks, which would play an important role in fulfilling the requirements of disaster prevention and reduction. Thus, this paper designs a monitoring technology for water body in disaster area on the basis of polarized GF-3 SAR images. Also, the lower reaches of Changjiang area of Poyang County, from July to August 2020, was taken as a case study, the spatial distribution of water bodies and the inundated area were extracted, the place of polders and risks were positioned, and their spatial relationships were analyzed and found. The research results can provide decision support for 3 S technology's application in flood control practice as well as effective technical support for flood control challenges in future.