In floodplain wetlands, alterations in hydrological patterns resulting from climate change and human activities could potentially diminish the carbon sequestration capacity of the soils, thereby having a negative impact on global climate change.  However, the magnitude of the influence of hydrological regime change on soil carbon remains inadequately monitored.  To address this research gap, we collected 306 upper layer (0–20 cm) soil samples from the Dongting Lake floodplain between 2013 and 2022.  The random forest (RF) algorithm was used to analyze the spatial distribution of soil organic carbon (SOC) in the upper soil layer of Dongting Lake floodplain and the impact of climate and hydrological changes in the past decade on surface SOC in the East Dongting Lake area was studied.  In 2022, the SOC concentration of the Dongting Lake floodplain upper layer soil ranged from 3.34 to 17.67 g kg^–1, averaging 10.43 g kg^–1, with a corresponding SOC density of (2.65±0.49) kg m^–2 and total SOC stock of 6.82 Tg C (2.87–13.48 Tg C).  From 2013 to 2022, the SOC concentration of the upper soil layer of the East Dongting Lake area decreased from 18.37 to 10.82 g kg^–1.  This reduction could be attributed to climate and hydrological changes which reduce SOC input by reducing vegetation growth and accelerating SOC decomposition.  Above 21.4 m elevation, the amount of SOC loss increased with elevation, the loss being related to the decline in Miscanthus community biomass and greater susceptibility of higher altitude areas to climate and hydrological changes.  Our results highlight the need for strengthening wetland SOC management to increase SOC in the soils to help combat climate change.