Lakes and river-related floodplains are hydrologically complex and dynamic systems that exhibit frequent wetting and drying. Poyang Lake and its extremely productive wetland constitute the largest lake-floodplain system of the Yangtze River basin. This study aims to use a daily water balance model in combination with a physically based hydrodynamic model to investigate the overall hydrological regime of the lake-floodplain system. Water balance analysis shows that 79.0% and 12.2% of yearly inflows are from river discharges from the upstream gauged and downstream ungauged catchments, respectively. The direct precipitation contributes around 3.0% on the lake surface, while the balance of 1.2% is sourced from floodplain runoff (0.5%) and lake's backflow (0.7%). Around 86.9% of the total lake outflow is discharged into the Yangtze River, while 1.5% evaporates for the lake water surface. Net groundwater discharge (11.6%) has greater impacts on the water balance than the net groundwater recharge (4.6%). Water balance results highlight that the catchment rivers and the associated groundwater system are important parts of Poyang Lake. In general, the catchment rivers exhibit higher flashiness during the rising and flood periods than the other periods, and the flashiness in the lake downstream and floodplains is higher than in the lake upstream regions and the main lake, respectively, demonstrating spatiotemporal variability in the flood pulse in the lake-floodplain system. This study contributes to provide more detailed information regarding hydrological components and their relative effects to decision-makers for both Poyang Lake and other similar floodplains, given proposals to cope with the climate and human interventions and the accelerating pace of water resources and water safety management.