Abstract: This study conducted a monthly investigation on the spatiotemporal characteristics of phytoplankton communities at 11 sites in the Qionghai Lake from May to November 2022. A total of 137 species of 76 genera and seven phyla were successfully identified. Green algae and diatoms dominated the composition of algae, and there were noticeable spatial and temporal differences in algae community structure. The density varied from 305.63×10⁴ to 706.91×10⁴ cells/L, averaging 442.76×10⁴ cells/L. Moreover, the density of algae in June was significantly higher than those in other months. Cyclotella sp., Chlorella sp. and Oocystis sp. were the dominant species every month. The water quality was generally good, corresponding to the Shannon-Wiene index, Simpson index, and Pielou evenness index ranging from 3.16 to 3.66, 0.82 to 0.88, and 0.66 to 0.75. The algae diversity was significantly higher from May to August than from September to October. At the same time, the comprehensive trophic level index was lower than 40, which was the medium nutrient level. In the northwest, the water quality in the Gaojian Bay was a light eutrophication level. The RDA analysis showed that the main environmental factors significantly affected the composition and distribution of alga community in the Qionghai Lake. Total phosphorus concentration had the most significant impact on phytoplankton, accounting for 31.1% (p<0.01) of the total variation rate, followed by dissolved oxygen (24.7%), water temperature (17.3%), and pH (10.6%). The Chlorophyll a concentration was negatively correlated with transparency (R=-0.65, p<0.01), while it was significantly positively correlated with permanganate index (R=0.35, p<0.01), ammonia nitrogen (R=0.43, p<0.01), total phosphorus (R=0.73, p<0.01), total nitrogen (R=0.39, p<0.01), and comprehensive trophic level index (R=0.35, p<0.01), respectively. These results indicated that the phytoplankton biomass in the Qionghai Lake increased with water nutrient concentration. In contrast, the biomass decreased with water transparency. Notably, the Shannon-Wiener index, Simpson index and Pielou evenness index were positively correlated with transparency and negatively correlated with permanganate index, ammonia nitrogen, total phosphorus, total nitrogen, and comprehensive trophic level index. On the whole, total nitrogen, ammonia nitrogen, water temperature, dissolved oxygen, permanganate index, and transparency were the main factors affecting the distribution of algal populations in the Qionghai Lake.