Abstract: Lakes play a natural role in trapping floodwaters; however, in the face of massive floods, the limited storage capacity of lakes can risk causing dyke overflow and dangerous situations in downstream areas. Therefore, it's essential to conduct causative analyses on floods and take timely measures to mitigate flood damages when conditions conducive to flooding arise. In 2020, the Poyang Lake basin experienced an unprecedented flood, with the water levels of the Yangtze River mainstream and Poyang Lake remaining high for an extended period. Taking the Poyang Lake basin as the study subject and based on the ground observation data from rainfall stations and hydrological stations within the basin in 2020, this study systematically analyzed the causes of the high water levels using statistical analysis methods. The results show that the increased inflow into Poyang Lake, due to precipitation in the basin and the runoff brought by the rainfall, is the primary reason for the formation of high water levels in Poyang Lake. In the early stages of the flood, the surge from the Yangtze River mainstream played a significant role in initiating and subsequently maintaining high water levels in Poyang Lake. The analysis of the causes behind the 2020 flood in Poyang Lake provides a theoretical basis and guidance for policy-making on the analysis of flood causes in river basins.