This paper was studied the eco-characteristics of phytoplankton in Lake Wuli, Lake Taihu in 2007. The results showed that 8classes including 123 species have been founded in Lake Wuli. Among them, dominated Chlorophyta has 57 species, accounting for46.3% of the total phytoplankton species, and Bacillariophyta has 23 species, taken the second place, accounting for 18.7% of the totalphytoplankton species. There were much more species in winter and spring than that in summer and autumn. The quantity and biomass ofphytoplankton in Lake Wuli in 2007 ranged from 386.2×10⁴to 5581.9×10⁴cells/L and 0.541 to 3.491mg/L, respectively. Dominated classboth in quantity and biomass was Chlorophyta, then was Cyanophyta, and the third was Cryptophyta. The quantity of phytoplankton inthe order from large to small was: Summer>Spring>Autumn>Winter. Seasonal succession of phytoplankton was consistent with rule ofPEG (Plankton Ecology Group) model except Chlorophyta. The analysis results of similarity showed that the eco-environment of LakeWuli in January was similar to that on March, April and May, and the eco-environment in June was similar to that in July, August,September and November. The analysis result of the predominance index showed that there were 14 predominant species includingChlorella vulgaris, Chl.amydomonas microsphaera, Chroomonas acuta, Microcystis aeruginisa in 2007 in Lake Wuli, and the numbers ofpredominant species were all more than two species in every month. Furthermore, the predominant indexes were not high, ranging from0.02 to 0.78. The analysis results of diversity and uniformity showed that the diversity index ranged from 1.5 to 2.7 and uniformity indexfrom 0.26 to 0.59, respectively, which meant that both diversity and uniformity of phytoplankton were good status in the lake. It could beconcluded that the phytoplankton community structure in Lake Wuli was complex in the ecosystem.