<p>Phytoplankton, as primary producers in aquatic ecosystems, serve as indicator organisms for eutrophication and are highly sensitive to changes in the aquatic environment. In subsidence lakes formed by underground coal mining in eastern China, both water quality and quantity undergo long-term dynamic evolution, presenting non-negligible eutrophication risks. To investigate the mechanisms of eutrophication in these lakes, this study examined phytoplankton communities in coal mining subsidence areas by selecting typical open and closed subsidence lakes. It identified spatiotemporal characteristics of eutrophication and phytoplankton variations, revealed correlations between phytoplankton community structure and environmental factors, and explored eutrophication drivers. Results indicated that the open subsidence lake exhibited moderate eutrophication year-round, while the closed subsidence lake were mildly to moderately eutrophic. A total of 166 phytoplankton species across seven phyla were identified, predominantly from Chlorophyta, Cyanophyta (cyanobacteria, prokaryotes), and Bacillariophyta. Cell density and biomass peaked in autumn and were higher in the open than the closed lake. Phytoplankton community structure correlated with dissolved oxygen (DO), nitrate nitrogen (NO₃⁻-N), pH, total phosphorus (TP), phosphate phosphorus (PO₄³⁻-P), and chlorophyll <i>a</i> (Chl.<i>a</i>). There is a dynamic interaction mechanism among environmental factors, eutrophication, and phytoplankton community in the subsidence lake studied. This research provides scientific basis for protecting aquatic ecosystems in coal mining subsidence lakes.</p>