Abstract: Dongting Lake (DTL), the second-largest freshwater lake in China, has undergone pronounced changes in its hydrological regime over recent decades due to climate change and human activities. This study employed non-parametric statistical methods to quantitatively analyze the inter-annual and intra-annual changes in DTL water levels, as well as the changes in its flood and dry seasons. Furthermore, the double-mass curve method was applied to quantify the relative contributions of climate change and human activities to these water level changes. The results indicate that: ① Over the past 40 years, water levels in DTL have declined significantly. Compared with the reference period (1980~2002), water levels during the change period (2003~2022) decreased notably. Specifically, West DTL experienced a decline of more than 0.5 m from July to November; South DTL showed a decrease of over 0.7 m from July to December; and East DTL also declined by more than 0.5 m from July to December, with the most pronounced decline exceeding 1.6 m in September to October.② Compared with the reference period, dry season days increased significantly during the change period, while flood season days decreased accordingly. South DTL showed the largest increase in dry season days (about 46 days), followed by West DTL (about 41 days) and East DTL (about 27 days). At the same time, West DTL exhibited the greatest reduction in flood season days (about 23 days), South and East DTL decreased by about 12 days and 4 days, respectively.③ Climate change led to an increase in water levels of 0.20 to 0.48 m, while human activities resulted in a decrease of 0.60 to 1.54 m. The relative contributions of climate change and human activities are 52.5% and -152.5%, respectively, indicating that human activities, particularly reservoir regulation, are the main cause of the decline in water levels.