The study of the fractions and distribution characteristics of organic phosphorus in the sediment of the water level fluctuating zone of Lake Nansi is conducive to revealing the transformation of phosphorus in the lake, and has important scientific significance for controlling the eutrophication of Lake Nansi. Based on the sediment of the water level fluctuation zone of Lake Nansi, the improved Hedley continuous grading extraction, ultraviolet-visible spectroscopy and three-dimensional fluorescence spectroscopy were used to characterize the structural characteristics and stability of organic molecules in the sediment, and to reflect the differences in the structure and stability of organophosphates in the water level fluctuating zone; and the Pearson correlation analysis and principal component analysis were used to determine the correlation of various fractions of phosphorus and other physical and chemical indicators. The results showed that the average content of total phosphorus (TP) in the sediment of the water level fluctuation zone of Lake Nansi was 679.90 mg/kg, and the proportion of organic phosphorus (OP) was 20.03%-45.69%. The contents and relative proportions of organic phosphorus in each occurrence form were as follows: Residual organic phosphorus (Residual-Po, 67.58%) > calcium bound organic phosphorus (HCl-Po, 16.61%) > iron/aluminium bound organic phosphorus (NaOH-Po, 7.62%) > potentially active organic phosphorus (NaHCO₃-Po, 5.97%) > weakly adsorbed organic phosphorus (H₂O-Po, 2.22%). The content and fractions of OP in the sediment in the water level fluctuation zone of Lake Nansi were mainly influenced by endogenous sources, mainly from the metabolism of endogenous microorganisms. The risk of sediment phosphorus release in the fluctuating water level zone of the west bank of the lake was higher than that of the east and south banks. Correlation analysis showed that there was a significant negative correlation between sediment pH and phosphorus content in the fluctuating zone, indicating that the increase in sediment alkalinity may lead to a higher risk of phosphorus release to water. There was a significant positive correlation between organic matter and OP (R²=0.719, P<0.01), indicating that organic matter may be an important carrier of OP. Principal component analysis showed that all phosphorus fractions in the sediment were positively correlated, indicating that all phosphorus fractions in the water level fluctuation zone had homology. The results can provide a scientific basis for controlling the release of endogenous phosphorus and the risk of eutrophication in Lake Nansi.