Phytoplankton was investigated monthly in Lake Luoma, northern Jiangsu Province from January to December of 2014 in order to study the structure of phytoplankton community and the bio-assessment of water quality. A total of 7 phyla, including 71 genera or species of phytoplankton were identified. Chlorophyta was the most abundance taxa and followed by Cyanophyta and Bacillariophyta. The phytoplankton abundance of the whole lake was ranged from 1.52×10⁶ cells/L to 7.89×10⁶ cells/L, and the biomass was ranged from 0.048 mg/L to 12.44 mg/L. Nonmetric multidimensional scaling analysis demonstrate that the phytoplankton structure has obvious seasonal succession pattern. Compared to the 20th century, dominant genera have changed obviously. For example, the indicator genera of eutrophication, such as Cryptomonas, Ankistrodesmus, Synedra, Anabaena and Pseudanabaena have increased significantly, which indicates the increasing eutrophication trend of Lake Luoma since the last 20th century. Lakes Nansi, Hongze, Shaobo, Baoying and Gaoyou are close to Lake Luoma and are all connected by the Grand Canal. Compared with the four nearby lakes, the phytoplankton abundance in Lake Luoma are higher than that in Lake Baoying. Based on the bio-assessment of water quality, Lake Luoma has been suffered a transform from oligosaprobic zone to β-mesosaprobic zone pollution, and its eutrophication degree is lower than that of Lakes Hongze, Shaobo, Gaoyou, and is similar with some zones of Lake Nansi.