To clarify trends in water quality in Chaohu Lake, 480 surface water samples were collected monthly from 8 sampling sites in the eastern and western Chaohu Lake regions from 2016 to 2020. The content of dissolved oxygen (DO), permanganate index (COD_Mn), ammonia nitrogen (NH₃-N), total phosphorus (TP), total nitrogen (TN), and Chlorophyll a (Chl-a) were analyzed, and the water quality of Chaohu Lake was comprehensively evaluated by a comprehensive nutrient status index method and a fuzzy clustering method. The average contents of TP and TN in Chaohu Lake decreased over time. The mean values of TP and TN ranged from 0.049 to 0.16 mg/L and 0.98 to 2.2 mg/L, respectively. The mean value of NH₃-N decreased from 0.27 mg/L in 2016 to 0.098 mg/L in 2020. The mean value of COD_Mn decreased by 22.96% compared with 2016. Based on the fuzzy clustering method and a comprehensive trophic state index, the water quality of the lake was increased from 2018 to 2020, and the comprehensive trophic state value decreased significantly, also indicating that the overall water quality of Chaohu Lake has improved since 2018. The results of this study could provide a reference for the prevention and control of water pollution in the Chaohu Lake Basin.