Abstract: From August 2016 to July 2017, a one-year survey was conducted on the composition and density of phytoplankton in the Xiongjiang section of Shuikou Reservoir. There were 90 species of phytoplankton in the investigated waters, which belonged to 7 phyla of phytoplankton. The chief phytoplankton were bacillariophyta and chlorophyta throughout the year. The average annual density of phytoplankton was 1.81 × 105 cells/L. From September to October, it was dominated by cryptophyta. From November to March, it was dominated by chlorophyta. From April to June, it was dominated by bacillariophyta. Algae density had two peaks throughout the year, in March and August respectively. The high incidence of low dissolved oxygen in water was observed from September to October. The average biodiversity index was 3.07. The water quality assessment was poor-medium nutrition, belonging to medium-light pollution.