Abstract: As the primary producers of aquatic ecosystems, phytoplankton communities play important roles in shaping ecosystem stability and function. In recent years, the excessive proliferation and algal bloom of filamentous cyanobacteria in many oligo-mesotrophic reservoirs have been reported more and more frequently. In order to explore the driving forces of dominant genus succession of phytoplankton and the mechanism of excessive proliferation of filamentous cyanobacteria in large reservoirs, the Qiandaohu Reservoir was chosen as the case study. The relationships between the variation of phytoplankton community structure and environmental factors during the period from 2017 to 2021 were studied. A total of 92 genera which belong to 8 phyla were identified in Qiandaohu Reservoir, which were mainly composed of Bacillariophyta, Chlorophyta and Cyanophyta. The results found that both the density and biomass of phytoplankton in Qiandaohu Reservoir showed a yearly increasing trend, especially with regards to cyanobacterial density which increased significantly. In 2021, the annual average density and biomass of phytoplankton reached 1.54×10⁷ cells/L and 2.23 mg/L, respectively, which increased by 161.02% and 52.74% compared with 2017. During the period from 2017 to 2021, the dominant genus of cyanobacteria shifted from being dominated by Microcystis and Dolichospermum to various filamentous cyanobacteria including Pseudanabaena, Leptolyngbya, Raphidiopsis, and Dolichospermum. The phytoplankton functional group (FG) analysis also showed similar result, with the main dominant functional group being various filamentous cyanobacteria H1, S1 and S_N instead of M and heterocystous filamentous cyanobacteria H1. During the monitoring period, the mean annual values of TN and TN/TP ratio in Qiandaohu Reservoir decreased significantly, while TP showed an increasing trend. The Mantel Test and SEM analysis indicated that the surface runoff input from rainfall contributed significantly to the increase in TP, and the increase in total biomass of phytoplankton was significantly correlated with TP, WT, DO, pH, SD and Temp. The total density of phytoplankton was mainly by TN, WT, pH, SD, Pre and Temp. Moreover, the changes in density and biomass of cyanobacteria were both driven by TN, WT, pH, SD and Pre. The redundancy analysis (RDA) showed that the changes of dominant genera and dominant functional groups were jointly driven by TN, TP and WT. The responses of filamentous cyanobacteria to nutrition differed among genera. The dominant genera Pseudanabaena, Raphidiopsis and Leptolyngbya which belong to FGs S1 and SN were all negatively related to TN but Dolichospermum which belong to FGs H1 were positively correlated with TN and TP. The decrease of TN concentration might be one of the main reasons for the increase of the number of dominant filamentous cyanobacteria species in Qiandaohu Reservoir. The results of this study suggest that algal community composition in oligo-mesotrophic reservoirs are more susceptible to change caused by extreme climates (rainstorm, elevated temperature, etc.), nutrient inputs and water disturbances. Intense rainfall could increase water mixing and nutrient pulse supply, which might trigger excessive proliferation of filamentous cyanobacteria in reservoirs. The response of filamentous cyanobacteria to environmental factors are complex, which need to be further studied in different genera. Our study can provide important implications for the prediction, prevention and control of cyanobacterial blooms in oligo-mesotrophic reservoirs.