Groundwater dynamics processes in floodplains affect the transfer and exchange of materials, energy, and information between hydrological environment systems and are important forcings for maintaining the integrity of the floodplain wetland ecosystem. This study focuses on the groundwater system in the floodplain area of Poyang Lake in the middle reaches of the Yangtze River. Based on spatial zoning of hydrogeological parameters and vertical layering characterization of aquifers, a qusi-3D groundwater flow numerical model of the Poyang Lake floodplain area was constructed by using FEFLOW. The study quantitatively analyzed the spatiotemporal response characteristics of groundwater dynamics under different hydrological stages in normal years, revealing the relative contribution components of the equilibrium factors in the floodplain"s groundwater system and the dynamic changes in groundwater storage throughout the year. The research shows that the model exhibits good predictive ability in simulating groundwater level changes over time and space. The determination coefficient of groundwater level simulation during the calibration period (2018) and validation period (2019-2020) was greater than 0.85, with a Nash efficiency coefficient greater than 0.80, and the groundwater level deviation was generally less than 0.45 m. Influenced by the annual dynamic water level changes in Poyang Lake, the groundwater level in the floodplain exhibits distinct seasonal fluctuations, with differences of about 2-3 m between the wet and dry seasons. Due to the heterogeneous topography and geomorphology of the entire lake basin, the groundwater level in the floodplain shows a spatially differentiated distribution pattern, with higher levels in the south, lower levels in the north, and higher levels in the east and lower levels in the west. Under the combined effect of lake water levels and topography, the groundwater flow velocity in most areas of the floodplain is less than 0.1 m/d. However, the groundwater flow velocity in some areas can reach up to 0.3 m/d during the rising water and wet seasons. The main flow direction of the groundwater is from the east, near the main lake area, toward the west floodplain, with this flow trend being particularly obvious during high water levels. The water budget analysis shows that precipitation infiltration, evaporation, and the water exchange between the aquifer and the lake area are the main factors affecting floodplain groundwater budget. Precipitation infiltration accounts for about 64%-65% of the total recharge volume, while lake water recharge accounts for about 29%-30%, and the infiltration recharge capacity of the seasonal lake group is relatively weak, contributing only 2% of the total recharge. Evaporation accounts for about 70%-73% of the total discharge volume, and the aquifer"s discharge to the lake accounts for about 23%-26% of the total recharge. In the spring and summer, the groundwater system in the Poyang Lake floodplain mainly shows positive balance (i.e., receiving water), while in the autumn and winter, it mainly shows negative balance (i.e., discharging water). The results of this study can provide a critical scientific basis and decision-making support for addressing seasonal droughts in Poyang Lake, ensuring regional water supply security, and sustaining wetland ecosystem.