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The phytoplankton population of Qionghai Lake was surveyed in December 2015, March 2016, June 2016, September 2016, and March 2017. A total of 196 species (including varieties) belonging to 77 genera of 7 phyla were identified. The phytoplankton communities were dominated by Chlorophyta and diatoms, and there were significant differences across the five sampling sites. The phytoplankton abundance, which ranged between 13.85 × 10⁴ and 335.54 × 10⁴ cells·L⁻¹, was significantly higher in spring and summer than in autumn and winter. <i>Chlorella</i> sp. and <i>Cyclotella</i> sp. were the dominant populations, and their dominance degree reached as high as 0.54 and 0.33, respectively. The diversity of the phytoplankton populations was significantly higher in spring and summer than in autumn and winter, and the Shannon–Wiener index and Margalef index ranged from 2.49–3.65 and 2.47–3.10, respectively. The water quality of Qionghai Lake was generally good. The trophic level index was between 30 and 60, showing that the water body was overall in a mesotrophic to slightly eutrophic state. The Spearman correlation analyses revealed that ammonium nitrogen (NH₄⁺-N), water temperature (WT), permanganate index (COD_Mn), and transparency (SD) were the most important environmental factors that influenced the phytoplankton communities. For example, NH₄⁺-N was significantly correlated with <i>Chroococcus</i> sp. (<i>r</i> = 0.41, <i>p</i> < 0.05) and <i>Cryptomonas ovata</i> Ehrenberg (<i>r</i> = 0.45, <i>p</i> < 0.05), and WT was significantly correlated with <i>Cryptomonas marssonii</i> Skuja (<i>r</i> = 0.43, <i>p</i> < 0.05) and <i>Cryptomonas ovata</i> (<i>r</i> = 0.53, <i>p</i> < 0.01).