To investigate the mobilization of iron and phosphorus in the sediments of reservoirs in southern China during the disappearance of seasonal thermal stratification, physicochemical indexes were in-situ monitored from November 2019 to January 2020 in shallow and deep area of Tianbao Reservoir. Simultaneously, diffusive gradients in thin films (DGT) technique were used to acquire the concentration distributions of labile Fe and P in the sediments. The results show: (1) The concentrations of labile Fe ranged from 4.67 to 18.72 mg/L, which far exceeded that of Lake Taihu, Lake Poyang and Lake Dongting, while the concentrations of labile P ranged from 0.003 to 0.073 mg/L. The concentrations of labile Fe in sediments of the shallow area were higher than those of the deep area. (2) The space-time differences between labile Fe and P in the sediments were obvious during the disappearance of thermal stratification. In the shallow area, the concentrations of labile Fe presented decline order: Day14>Day1>Day32>Day54, and labile P was Day1>Day14>Day32>Day54, while the concentrations of labile Fe and P increased in the sediments of the deep area. It was attributed to the quick response of thermal stratification in shallow area to the sudden drop in temperature, consequently increasing the release of Fe and P at the sediment-water interface. (3) The diffusion flux of labile Fe at the sediment-water interface ranged from 6.58 to 31.59 mg/(m²·d), and that of labile P ranged from 0.008 to 0.086 mg/(m²·d), which meant Fe and P diffused from the sediments to the overlying water. The mixing time in the shallow area was earlier than the deep area, resulting in the significant contribution of internal Fe and P release to deterioration of water quality in winter. Therefore, the internal release of sediments in the shallow area should be paid more attention in reservoirs of southern China.