The dynamic changes of phytoplankton in rivers can better indicate the water quality of rivers. In order to explore the effects of hydro-meteorological processes on the phytoplankton community structure of damming rivers, taking the main stream of Qiantang River as an example, the phytoplankton community structure and related environmental factors were investigated and analyzed in 11 river reaches including Fuchunjiang Reservoir in the summer of 2020. The results showed that:A total 95 species belonging to 6 phyla and 59 genera of phytoplankton were identified during the summer in the main stream of Qiantang River. The dominant genera were Aulacoseir, Nitzschia, Merismopedia, Dolichospermum, Pseudoanabaena, and Microcystis. During the Meiyu period, the dominant genera belonged to the diatoms, while after the rain, the cyanobacteria quickly became dominant. The average cell density of phytoplankton in summer was (1.57±2.96)×10⁶ cells/L, and the cell density during Meiyu period was (0.37±0.36)×10⁶ cells/L, which was significantly lower than that in Meiyu period ((4.06±4.38)×10⁶ cells/L). Spatially, the river reservoir formed by dam interception has an extremely significant effect on phytoplankton community structure and algal density. The phytoplankton density is the highest in the Fuchunjiang River reservoir section, followed by the Lanjiang River and its upper reaches, and the Xin'anjiang River reservoir section and the Qiantang River estuary. After the Meiyu period, the water temperature stratification in the Fuchunjiang River reservoir significantly promoted the proliferation of cyanobacteria and diatoms. Redundancy analysis showed that temperature and rainfall were the main driving factors of phytoplankton community structure change in the main stream of Qiantang River in summer. Cyanophyta and Chlorophyta cell densities were positively correlated with temperature and negatively correlated with rainfall, while Diatomophyta cell densities were contrary to Cyanophyta. The rapid increase of temperature after the Meiyu period can rapidly increase the cell densities of cyanobacteria in the reservoir area, and the risk of cyanobacteria blooms increases rapidly. Therefore, the prevention of cyanobacteria blooms should be emphasized in water safety management.