Reservoirs in southwestern China are encountering the challenge of eutrophication, in which internal phosphorus (P) release from sediments plays an important role. Studies on the high-resolution profile variations and release mechanisms of P at the sediment-water interface (SWI) are rare in these reservoirs until now. In this study, monthly monitoring (Nov 2017 to Oct 2018) using a composite diffusive gradient in thin films (DGT) technique was taken to determine the temporal and vertical profile variations of the DGT-labile P (Fe) at the SWI in Hongfeng Reservoir. The results showed that the average concentrations of the DGT-labile P (Fe) in surface sediments were 0.63 ± 0.24 mg·L⁻¹ and 4.61 ± 1.12 mg·L⁻¹, respectively, with significantly higher concentrations during the summer anoxic period than that during the winter aerobic period. The DGT-labile P (Fe) concentrations in sediments presented a significant positive correlation (r² > 0.70, p < 0.001), supporting the simultaneous release of reactive P and reactive Fe from the sediments and indicating that the reductive dissolution of iron-bound P dominates the P release from sediments. The release rates of P ranged from 0.01 mg·m⁻²·d⁻¹ to 0.83 mg·m⁻²·d⁻¹ (mean: 0.22 mg·m⁻²·d⁻¹) in Hongfeng Reservoir, which are higher than that in heavily eutrophic shallow lakes in eastern China, such as Lake Taihu. There is a higher P loading, stronger P reactivity, faster P release rates, and higher pollution potential in deep reservoirs of southwestern China than that in natural shallow lakes of eastern China, highlighting the importance and urgency of treating internal P pollution in deep reservoirs. Further studies on the mechanisms and controlling factors of the coupled Fe-P-S cycle in deep reservoirs are desirable in the future, so as to provide a scientific foundation for exploring effective internal P treatment techniques adaptive to deep reservoirs in southwestern China.