Abstract: As one of the severely eutrophic lakes in the Changjiang River Basin, Dianchi Lake faces long-term challenges in water quality improvement and ecological restoration. To accurately diagnose its current eutrophication status, this study systematically examined the spatiotemporal characteristics from 2021 to 2023 and interannual trends of key water quality and trophic level indicators in Dianchi Lake —namely chlorophyll-a, total phosphorus (TP), total nitrogen (TN), and oxygen-consuming organic pollutants (represented by COD_Mn). The trophic level of Dianchi Lake was assessed using the single-indicator method based on chlorophyll-a, and the quantile method was applied to determine nitrogen and phosphorus reference conditions. The results indicated that: ① from 2021 to 2023, the water quality of Dianchi Lake was predominantly Class Ⅳ to worse than Class Ⅴ, with TP, TN, and COD_Mn being the main parameters exceeding the standards. The average TP concentrations in Caohai and Waihai Lake were similar, measuring (0.067±0.001) mg/L and (0.064±0.004) mg/L, respectively. In contrast, the average TN concentration in Caohai Lake ((4.19±1.10) mg/L) was significantly higher than that in Waihai Lake ((1.54±0.09) mg/L). The average COD_Mn concentration in Caohai Lake ((4.09±0.66) mg/L) was lower than that in Waihai Lake ((6.67±0.66) mg/L). The average chlorophyll-a concentrations in Caohai and Waihai Lake were (106±22) μg/L and (77±6) μg/L, respectively. From March to December each year, both the mean and median chlorophyll-a concentrations across all areas exceeded the severe eutrophication threshold of 30 μg/L. During January and February, chlorophyll-a concentrations were relatively lower across the entire lake, with an average of 29 μg/L. ② Dianchi Lake was in a state of severe eutrophication, with most areas experiencing algal blooms throughout the majority of the year. TN concentrations were lower during the wet season compared to the dry season, while TP and COD_Mn concentrations were higher during the wet season. Over the past decade, nitrogen, phosphorus, and oxygen-consuming organic pollutant levels in Dianchi Lake have shown a significant declining trend, reflecting notable achievements in pollution control. The effectiveness of TP control has been greater than that of TN control, and the nitrogen-to-phosphorus ratio has exhibited an upward trend. The current average N/P ratio in Dianchi Lake is 37, indicating excessive levels of nitrogen, phosphorus, and oxygen-consuming organic pollutants relative to the requirements for algal growth. Water pollution prevention and control should continue to be strengthened, and the control of TN should not be overlooked. The key to reducing nitrogen and phosphorus pollution in the lake lies in controlling the pollutant loads from its inflow tributaries. Nitrogen and phosphorus control targets for key monitoring sections of these tributaries should be established from the perspective of the aquatic ecological security of Dianchi Lake.