Abstract: The study adopts the digital elevation model (DEM) with 25 m resolution, and collects multiyear flood and tide level data to ensure the accuracy and reliability of source data. Combined with the geomorphic features and water system distribution in the study area, on the basis of ArcGIS spatial analysis technology, the seed spread algorithm is used for calculation of source flood submerge area, which obtains the submerged area and distribution of water depth. The results show that the submerged area is mainly distributed in reservoir, as well as alluvial-diluvial plain and marine plain in lower reaches of river, covering a total area of 318.08 km2 and accounting for 18.6% of land area of the whole district. The flood submerged depth is 0-7.6 m, and the submerged area with reservoir as the seed point is large, with the submergence depth gradually increasing from seed point to coastline, while the submerged area with river as the seed point is in zonal distribution, with the submergence depth decreasing gradually from middle line to both sides of river.