Phytoplankon samples were collected monthly in Dianshan Lake, the largest natural freshwater lake in Shanghai during 2004-2006, to assay the phytoplankton community structure and ascertain the relationship between its succession and the eutrophication for the lake. Two hundred and five species from 84 genera were identified in Dianshan Lake, with Chlorophyta (50%), Bacillariophyta (20%), Cyanophyta (13%) and Euglenophyta (13%) being the main phyla. The monthly similarity index of species was high in winter and spring, while low in summer and autumn. The dominant species were Merismopedia glauca,Phormidium tenus, Microcystis aeruginosa, M. marginata, Lyngbya limnetica, Chroococcus minutus, Melosira granulatavar.angustissima, Cryptomonas erosa, Chlorella vulgaris and Scenedesmus quadricauda. The total phytoplankton abundance was mainly composed of Cyanophyta (36.75%), Chlorophyta (16.78%), Bacillariophyta (16.36%) and Cryptophyta (13.53%). The total phytoplankton biomass was dominantly made up of Bacillariophyta (36.75%), Cyanophyta (16.78%), Chlorophyta (16.36%) and Cryptophyta (13.53%). The seasonal succession of phytoplankton community structure in Dianshan Lake was different from the PEG Model, with Cyanophyta of great abundance occurring at the end of spring, then dominating in summer and the early autumn. In the past five decades, the succession pattern of phytoplankton community in the Dianshan Lake showed a decrease in the proportion of such oligotrophic species as Chrysophyta and Dinophyta and an increase in such eutrophic species as Cyanophyta, Cryptophyta and Chlorophyta. The abundance increased from 10³ ind./L in 1959 to 1.11×10⁷cells/L during 2004-2006. The phytoplankton community structure changed from Bacillariophyta-Chrysophyta type in 1959, Bacillariophyta-Cryptophyta type during 1987-1988 to Cyanophyta-Chlorophyta type during 2004-2006. The changes in the abundance and structure of phytoplankton community and its seasonal succession indicated the evolving eutrophication of Dianshan Lake.