Abstract: Bosten Lake, the largest inland freshwater lake with inflow and outflow in China, plays a pivotal role in regional water-resource utilization and ecological balance. To elucidate the potential impacts of internal phosphorus (P) loading on the lake aquatic environment, we established 23 sampling sites both in the lake and in the principal inflow river inlets in April 2024. We investigated the basic physicochemical properties and the total phosphorus (TP) content in the sediments, and employed the traditional chemical extraction method along with high-resolution thin-film diffusive gradients technology (DGT) and dialysis techniques to characterize P fractions in sediments and their dynamic behavior at the sediment-water interface. We also applied the multivariate statistical analysis to elucidate the potential release patterns and influencing factors of sediment-bound P. The results showed that: (1) TP in surface sediments ranged from 264.26 to 719.23 mg/kg, with concentrations at the Kaidu River inlet significantly exceeding those in the main lake region and at the Huangshui River inlet (p<0.05). Among the various P fractions, calcium-bound inorganic P (Ca-Pi) exhibited the highest average content and an extremely strong correlation with TP (r=0.950), indicating that the TP distribution in the sediments was strongly influenced by Ca-Pi. (2) Consistent with TP, the mean bioavailable P concentration at the sediment-water interface determined by DGT was the highest at the Kaidu River inlet, followed by the Huangshui River inlet and the main lake region. Vertical fluctuations and regional differences in the P resupply coefficient (Kaidu River inlet > Huangshui River inlet > the main lake area) both confirmed that the sediments at river inlets exerted a stronger P replenishment effect on the water column. (3) The sediment P chemical availability showed an opposite trend to the sediment-water interface P release flux calculated by DGT, which indirectly indicated the transformation of mobile P into the water phase. The sediments at the Kaidu River inlet, characterized by the elevated content of potential active inorganic P, Fe/Al-bound inorganic P, and fine particle fraction, created favorable conditions for internal P release, thus driving the peak P diffusion flux to 157.05 ng/(m²·d). Overall, the study reveals that fine-grained sediments enriched with reactive P fractions are the primary drivers of internal P release in Bosten Lake, providing theoretical support for the control of lake eutrophication.