Dawangtan Reservoir is a typical large drinking water source reservoir in the south subtropical region. The dominance of cyanobacteria and blooms due to eutrophication are major threats to water supply security. Therefore, elucidating the phytoplankton community characteristics and identifying the driving factors of filamentous cyanobacteria dominance hold imperative significance for developing effective bloom mitigation strategies and enhancing reservoir water management protocols. This study conducted quarterly investigations on the water physical and chemical environment and phytoplankton community of the Dawangtan Reservoir from December 2021 to October 2022, aiming to explore the seasonal dynamics of the phytoplankton community and the driving factors behind the dominance of filamentous cyanobacteria. The results indicate that the phytoplankton in Dawangtan Reservoir comprises 7 phylum and 127 species, predominantly belonging to Chlorophyta, Bacillariophyta and Cyanophyta, and can be classified into 26 functional groups. The cell density of phytoplankton ranges between 1.2×106 cells/L and 430×106 cells/L, with the highest density observed in Autumn, Spring, Winter, and Summer. The biomass ranges from 0.14 mg/L to 51 mg/L, with the highest biomass in Autumn, Winter and Spring, and the lowest in Summer. The dominant genera are filamentous cyanobacteria such as Limnothrix and Pseudanabaena, and the S1 functional group is the long-term dominant functional group. The comprehensive trophic level index of the Dawangtan Reservoir ranges from 36.02 to 49.57, demonstrating that the reservoir is characterized by a Mesotrophic status. Mantel tests and Redundancy analysis (RDA) revealed that water temperature, transparency, and nitrogen concentration are significant explanatory variables for the dominance of filamentous cyanobacteria. The absolute dominance of filamentous cyanobacteria was observed during the Summer and Autumn, characterized by lower transparency, reduced nitrogen concentration, and elevated water temperatures. Among them, Autumn may be a high - risk period for algae blooms of Limnothrix and Pseudanabaena. Combined with the functional group, the turbidity environment of large reservoirs is an important reason for the dominance of filamentous cyanobacteria. In Autumn, water temperature and nutrient conditions were in the suitable range for Limnothrix and Pseudanabaena, which were the key driving factors for their large-scale proliferation. Under the threat posed by filamentous cyanobacteria such as Limnothrix and Pseudanabaena, attention should be directed toward investigating the causative factors of elevated turbidity and chromaticity in reservoir water bodies. Enhancing water transparency through targeted interventions is critical to suppress the dominant formation of these filamentous cyanobacterial species.