Abstract: To explore the impacts of the South-to-North Water Diversion Project on the change of planktonic diatom abundance and community in Nansi Lake, the diatom community and water quality were analyzed through monthly sampling from May 2010 to April 2013. The similarity of the diatom community from different sampling sites and seasons was analyzed by non-metric multi-dimensional scaling. Redundancy analysis and Pearson correlation analysis were applied to analyze the relationships between the diatom community and environmental factors. The results showed that the diatom community contained 23 genera and 69 species, and diatom abundance ranged from 14.3×10⁴ to 663.2×10⁴ L-1, being greater in winter and spring than in summer and autumn. The diatom community showed an obvious seasonal succession; Synedra acus was the dominant genus in winter for 15.3% and spring for 24.0%, while Cyclotella comta, which represented warm assemblages, was dominant in summer for 17.8% and autumn for 19.1%. Nitrogen and phosphorus nutrients, pH and temperature were the main environmental factors affecting the change of the diatom community. The diatom community in Nansi Lake showed sensitivity to the change in environmental factors, and is suggested to be monitored over the long term as a potential indicator.