Abstract: In order to provide a scientific basis for the ecological protection of Luoma Lake, northern Jiangsu Province, in the summer of 2017, the phytoplankton community in the lake was investigated to explore the relationship between phytoplankton community structure changes and environmental factors. A total of 6 phyla, including 32 genera of phytoplankton were identified. The number of Chlorophyta was the largest, followed by Cyanophyta and Diatoma, and the dominant genera were mainly Planktothrix, Microcystis and Chlorella. The phytoplankton abundance of the whole lake was ranged from 2.63×10⁵ to 2.85×10⁷ cells·L^-1, and biomass was ranged from 0.092 to 4.522 mg·L^-1. Shannon-Wiener index of phytoplankton in the whole lake ranged from 0.60 to 2.60, with an average of 1.75, moreover, the index in September was significantly higher than that in August (P < 0.05), but the difference in diversity index between different locations was no significant (P>0.05). Compared with the results of previous investigations in Luoma Lake, the diversity index of the whole lake did not have much difference, the cell abundance was significantly increased, and the dominant genera were concentrated in Cyanophyta and Chlorophyta, especially the number of Planktothrix increased distinctly. From the changes of dominant genera and algae cell density in recent years, the eutrophication degree of Luoma Lake has still been increasing. The results of RDA analysis show that the combination of water temperature, dissolved oxygen and nitrogen-phosphorus ratio explained 28.16% of the phytoplankton community structure change in the summer of Luoma Lake. Sand mining, purse seine culture and summer human activities had a comparatively large impact on this change. Therefore, it is imperative to reduce the disturbance of human activities to protect the water quality of Lake Luoma and the water quality of the South-to-North Water Transfer Project.