The community structure of Rotifera and ecological assessment of water quality, including species composition, population dynamics, standing crops and indicator organisms, E/O and Q_B/T index, was studied in a large, deep lake, Qianda Lake, Zhejiang Province. After one-year monitoring in situ, 70 species of Rotifera were found, 41 species were pollution indicator species, of which oligo-β mesosaprobic, mesosaprobic and β-α mesosaprobic species accounted for 415%, 36. 5% and 22. 0% of the total indicator species, separately. In terms of annual average density, the dominant species were Keratella cochlearis, Trichocerca similis and Polyarthra trigla, which accounted for 59.2% total density. Asplanchna spp. accounted for 56.5% of total biomass. The annual average density of Rotifera was 103 ind./L, and biomass was 0.069mg/L. Except for Keratella cochlearis, which form stable population, the density distribution of other dominant species were varied temporarily from station to station, such as Schizocerca diversicornis density reached 800ind./L at station Ⅱ in May, Trichocerca similis density reached 1000ind./L at station Ⅲ in July, and Keratella cochlearis density reached 560ind./L at Ⅵ station in September. Biomass changes in space and times were determined by Asplanchna spp, the higher biomass at station Ⅰ and Ⅱ in May, reached 0.169 mg/L and 0.275 mg/L, separately. The ranges of E/O values in Qiandao Lake were 0.29-0. 62(0.46 on average), and the ranges of Q_B/T values were 0.17-1.50(0.68 on average). The regression analysis showed that the transparency of lake water had negative linear relationships with E/O value (r²=0. 7182) and Q_B/T value(r²=0.7747). The characteristics of Rotifera community in Qiandao Lake showed the rich species number and low density, biomass and Q_B/T value compared with other large shallow lakes in China. Furthermore, biological assessment of water pollution and trophic condition were evaluated by species indicators, biotic index of E/O and Q_B/T. The trophic type of Qiandao Lake could be regarded as oligo- mesosaprobic, of which station, Ⅰ, Ⅱ and Ⅴ were considered as β-mesosaprobic, while others were considered as oligo- mesosaprobic.