Abstract: In order to understand the characteristics of phytoplankton community structure and biomass and their effect on water quality of Dongfeng Reservoir, three samples of phytoplankton were collected every month from June 2019to May 2020, the annual variation characteristics, dominance, diversity and evenness indices of phytoplankton were analyzed by species identification and density statistics. The results showed that a total of 42 genera of phytoplankton identified, belonging to 8 Phyla in the water samples from Dongfeng reservoir. Chlorophyta was the dominant Phyla, the highest in general, and followed by the Phyla of Cyanophyta and Bacillariophyta. Density of the phytoplankton varied in the range from 73.25×10⁴ to 14678.67×10⁴ Cells/L(5576.43×10⁴ Cells/L in average). The indices of Shannon-Wiener and Pielou evenness and Margalef ranged from 0.54 to 2.69(1.41 in average) and ranged from 0.13 to 0.67(0.35 in average) and ranged from 0.93 to 2.78(1.48 in average). According to Shannon-Wiener Diversity and Pielou evenness and cell density index to assess the water quality of Dongfeng Reservoir, the results indicated that the water quality was moderately polluted and in medium eutrophication.