Abstract: A whole basin catastrophic flood occurred in the Changjiang River during July-August, 1998.To obtain insight into the effect of the flood on aquatic plants, the biomass and density of three dominant species, i.e., Potamogeton malaianus, Miq, Vallsineria spiralis Linn and Carex spp, in Poyang Lake were analyzed.At the corresponding period of time before and after the flood, biomass and density of aquatic plants changed significantly (p 0.01).It was suggested that the flood was the main reason for the changes.The effect of the flood on aboveground part of aquatic plants was more serious than on the underground part, and the most serious effect was on submerged plants.Aboveground part of submerged plants died, only few subterranous stems (vegetative propagules) of them remained alive after the flood, 3) the flood influenced the recovery of aquatic plants in 1999, and would lead to lower primary production of the lake.