The dynamic changes of phytoplankton community and its relationship with major environmental factors are important for understanding the changes of lake and reservoir water environment and carrying out water environmental quality assessment. In this study, a monthly survey of phytoplankton and water environmental factors in Qiandao Lake was conducted from 2021 to 2022, and the characteristics of phytoplankton community structure, spatial and temporal distribution pattern and main environmental driving factors were analyzed, in order to provide important basic data for understanding the latest water environment changes and ecosystem evolution in Qiandao Lake. The results showed that a total of 240 species belonging to 8 phyla were identified, with Chlorophyta (51.67%), Bacillariophyta (21.67%) and Cyanophyta (17.84%) as the main species, and Cyanophyta and Bacillariophyta as the main dominant species. The average annual biological density from 2021 to 2022 was (404.21±116.59)×10⁴ cells/L, and the average annual biomass was (0.84±0.22) mg/L, showing significant spatio-temporal differences in both biological density and biomass. Cyanophyta had the highest biological density and biomass of phytoplankton. Shannon diversity index showed significant seasonal differences, but no significant spatial differences, and there was no spatio-temporal difference in other indices. Spearman correlation analysis showed that phytoplankton density and biomass were significantly positively correlated with water temperature, ammonia nitrogen and permanganate index, and negatively correlated with transparency. RDA analysis showed that water temperature, ammonia nitrogen and nitrite were the main environmental factors affecting the distribution of phytoplankton dominant species in Qiandao Lake.