Abstract: To elucidate the aquatic environmental characteristics and their impact on phytoplankton communities in oxbow lakes situated within the middle reaches of the Yangtze River, investigations and analyses were conducted for both before and after the flood seasons. Water quality indicators encompassed TP, TN, NH₃-N, COD_Mn and BOD₅. Phytoplankton investigations involved species composition, biomass, and biodiversity assessments. Significant indicators influencing lake water quality were identified, and a quantitative evaluation of water quality pre and post flood seasons was executed by constructing an optimized fuzzy mathematical evaluation method. Additionally, the eutrophication status of each lake was assessed. Grey relational analysis and RDA analysis were employed to pinpoint the primary water quality indicators impacting the characteristics of phytoplankton communities. Several key conclusions were drawn. (i) The water quality of oxbow lakes in the middle reaches of the Yangtze River generally ranged between III and IV. Lakes controlled by water gates exhibited better water quality in flood seasons than those freely connected to the Yangtze River, while the reverse was observed post-flood seasons. (ii) The primary water quality indicators influencing phytoplankton density were TP and COD_Mn , while TN, NH₃-N, and COD_Mn were the main indicators affecting the diversity of phytoplankton communities. Oxbow lakes were typically in a mesotrophic state before flood seasons, and there was a slight increase in eutrophication for post-flood seasons. (iii) Implementing measures including nutrient control, water level regulation, and hydraulic control could aid in restoring lake water quality and optimizing phytoplankton diversity and richness.