The investigation of the aquatic vegetation in Dianchi Lake has been carried out from 1995-1997.This paper described the sepcies composition, community structure, distribution, biomass and succession dynamics of aquatic vegetation in the Lake, and analyzed the interaction between the change of the aquatic vegetation and the environmental quality.Comparing with over 100 species found in the 1950s in Dianchi Lake, there are only 22 sepcies of auqatic vascular plants collected from this lake, among which Eichhornia crassipes, Alternanther phiLoxeroides, Pota-mogeton malainus and Myriophyllum spicatum are the dominant species off aquatic vegetation.The aquatic vegetation of Dianchi Lake from 1995 to 1997 could be divided into 9 associations.Due to the eutrohpication and the water pollution, the area of the aquatic vegetation decreased from 90% to 1.8% and the biomass decreased from 1363.1 g m^-2 to 136.73 g m 2during tl.last 50 years.The community structure has simplicated and deteriorated drastically, the biodiversity changed periodcally along with the succession of the major species.The Dianchi Lake has completed the transition from submerged-plant-dominant-type, and Caohai has being in the third alternative stable state which the floating plant is dominant.