Abstract: Tongshanyuan Reservoir is a storage reservoir in Quzhou City, Zhejiang Province, which plays an important role in agricultural irrigation and hydroelectric power generation. In order to understand the characteristics of the phytoplankton community structure and its relationship with water environmental factors, water samples were collected from Tongshanyuan Reservoir in March, May and July 2021. The results show that 92 species of phytoplankton belonging to 7 phyla and 57 genera were identified. The dominant species were Cyclotella sp., Eudorina sp., Coelastrum reticulatum, Microcystis spp. and Aphanizomenon flos-aquae. Principal coordinate analysis (PCoA) show that the phytoplankton community structure was significantly different over the three mouths. The seasonal succession of the phytoplankton community structure in Tongshanyuan Reservoir changed mainly from Bacillariophyta/Chlorophyta to Cyanophyta and to Chlorophyta/Cyanophyta. Shannon-Wiener diversity index (H') indicate that the reservoir was moderately polluted. The abundance and biomass of phytoplankton decreased in May, but rose again in July. Canonical correspondence analysis (CCA) show that water temperature, total nitrogen and total phosphorus were the main environmental factors affecting the phytoplankton community structure in the Tongshanyuan source. The top-down effect of zooplankton was stronger in May. But the density and biomass of zooplankton had no significant effect on the changes in phytoplankton community structure. This study provides reference and basic data for understanding and maintaining the ecology of Tongshanyuan Reservoir.