The environmental restoration of Lake Baiyangdian is of great significance for regional construction of ecological civilization. However, information on the internal loadings of nitrogen and phosphorus, which aggravated the eutrophic status of Lake Baiyangdian, is still limited. In this study, a detailed field survey was carried out to investigate the occurrence and fractions of nitrogen and phosphorus in the sediment, the profiles of nitrogen and phosphorus in interstitial water, and the exchanges of nitrogen and phosphorus across the sediment-water interface. A series of in situ sediment core samples were collected in important water areas of Lake Baiyangdian for the study. Results showed that the contents of total nitrogen and total phosphorus in the sediments were in the range of 1230.8-9559.0 mg/kg (average:2379.5 mg/kg) and 344.4-915.4 mg/kg (average:608.4 mg/kg), respectively. A large quantity of the cumulative pollution of nitrogen and phosphorus was discovered. The high levels of ammonium nitrogen in the sediments (3.2-175.8 mg/kg) led to high concentrations of ammonium nitrogen in the interstitial water, with the highest concentration of 28.8 mg/L. The phosphorus forms in the sediments were largely Ca-P and Fe-P, accounting for 38.3%-76.1% and 3.98%-18.0%, respectively. The phosphate concentrations in the interstitial water were close to or even higher than those in typical eutrophic lakes around the world. The high levels of ammonium nitrogen and phosphate in the interstitial water resulted in high exchange fluxes of nitrogen and phosphorus across the sediment-water interface. The average release and diffusive fluxes of ammonium nitrogen were 106.37 mg/(m²·d) and 12.42 mg/(m²·d), respectively. The average release and diffusive fluxes of phosphate were 15.06 mg/(m²·d) and 2.33 mg/(m²·d), respectively. The high internal loadings of nitrogen and phosphorus in the sediments are now serious threat to the water environmental quality of Lake Baiyangdian. Urgent measures should be carried out to reduce the internal loadings. Especially, the northern estuary area and the central heavily populated area under the influence of the Fuhe River had higher internal loadings of nitrogen and phosphorus than other areas. These two areas should be given top priority during the remediation of internal loadings in Lake Baiyangdian.