Miyun Reservoir is a typical deep-water drinking water source reservoir in northern China. To explore the release potential of phosphorus in sediment of deep-water source reservoirs and its influencing factors and to compare the changes of endogenous phosphorus in Miyun Reservoir after the south water enters Beijing, this paper takes Miyun Reservoir as the research object, selects representative points to collect sediment and overlying water samples, and uses the improved continuous grading extraction method to study the occurrence form composition of phosphorus in sediment, and further studies the phosphorus exchange characteristics of sediment-water interface and its response to environmental factors through indoor simulation experiments. The results showed that the total phosphorus (TP) content of sediment in the reservoir area ranged from 533.1 to 2650.9 mg/kg, with high spatial heterogeneity. Compared with previous studies, the phosphorus content of sediment in the reservoir area has an upward trend. According to the distribution of total phosphorus concentration between overlying water and interstitial water, sediment is the source of internal phosphorus load. The content of inorganic phosphorus in the sediment of the reservoir area was as follows: calcium phosphorus (Ca-P_i)＞iron aluminum phosphorus (Fe/Al-P_i)＞potential active inorganic phosphorus (PA-P_i)＞weakly adsorbed inorganic phosphorus (WA-P_i); the content of organic phosphorus was as follows: moderately active organic phosphorus (MA-P_o)＞inactive organic phosphorus (NA_NaOH-P_o)＞potentially active organic phosphorus (PA-P_o)＞weakly adsorbed organic phosphorus (WA-P_o). Ca-P_i had the highest content among all forms of phosphorus, accounting for 72.36% of inorganic phosphorus and 42.52% of total phosphorus, but its release risk was low. The content of Fe/Al-P_i and MA-P_o is second only to Ca-P_i and can be further released into water under the influence of many environmental biological conditions, which can be considered as the main source of phosphorus release from sediment in Miyun Reservoir. The phosphorus exchange flux at the sediment-water interface of Miyun Reservoir was 0.014-0.022 mg/(m·h), and the overall exchange flux in the Chaohe zone was higher than that in the Baihe zone. Compared with other lakes and reservoirs in China, the sediment phosphorus exchange flux of Miyun Reservoir is lower, but the sediment TP content is relatively higher. In the environmental factors experiment, the exchange of phosphorus at the sediment-water interface of Miyun Reservoir is closely related to the concentration of PO₄ in the overlying water, temperature, pH and biological activities. Through field measurement, the concentration of PO₄ in the overlying water of Miyun Reservoir is low, and the sediment still has high phosphorus release potential under the promotion of sediment-water interface concentration gradient. The phosphorus release from sediment in shallower water areas is more susceptible to temperature rise in summer. Under the existing conditions, the pH at the sediment-water interface has little effect on the release of phosphorus. Phosphorus showed a significant release trend under the influence of biological activities, and attention should be paid to the number of algae, microorganisms, and benthic organisms in Miyun Reservoir.