Abstract: Phytoplankton are the most vital primary producers in lake ecosystems, and their community structure reflects dynamic shifts in lake ecological conditions. To characterize the phytoplankton community structure and its driving factors in Fenhe Second Reservoir, phytoplankton and water samples were collected and analyzed in July (summer) and November (autumn) 2024. The results indicated that a total of 68 phytoplankton species belonging to 45 genera and 7 phyla were identified in summer, while 50 species covering 39 genera and 6 phyla were detected in autumn. The phytoplankton cell densities were 13.58×10⁶ cells/L and 4.47×10⁶ cells/L in the two seasons, respectively. The comprehensive trophic level index (TLI) of Fenhe Second Reservoir was 50.38 in summer, indicating mild eutrophication of the water column, while the TLI reached 47.38 in autumn, placing the reservoir in a mesotrophic state. Among Bacillariophyta (Cyclotella meneghiniana), and among Cyanophyta(Phormidium tenue), were the absolute dominant species in both summer and autumn. Chlorophyta served as the dominant taxon in summer (39.71%), whereas Bacillariophyta prevailed in autumn (40.00%), collectively, Bacillariophyta and Chlorophyta dominated the phytoplankton assemblages throughout both seasons. The Margalef richness index was markedly higher in summer than in autumn, whereas the Shannon-Wiener diversity index and Pielou evenness index were higher in autumn, the diversity indices suggested that Fenhe Second Reservoir was under slight to moderate pollution stress. Pearson correlation analysis demonstrated that surface water temperature (ST), Secchi depth (SD), dissolved oxygen (DO), ammonia nitrogen (NH₃-N), phosphate, chlorophyll a (Chl-a) and permanganate index were critical environmental variables affecting phytoplankton density and biomass across summer and autumn. Redundancy analysis indicated that despite divergent driving factors regulating phytoplankton community structures in summer and autumn, the community dynamics were collectively driven by ST, NH₃-N, SD, DO, phosphate and Chl-a. Therefore, changes in nutrient concentrations and organic matter content should be prioritized in subsequent aquatic ecological conservation practices for Fenhe Second Reservoir.