As an important storage hub of the Eastern line of the South-to-North Water Transfer Project, the periodic fluctuation of water level in Lake Nansi leads to the formation of a large-area water-level-fluctuating zone (WLFZ), which affects the water quality of Lake Nansi. Understanding the distribution characteristics, influencing factors and release potential of phosphorus in the sediment of the WLFZ of Lake Nansi is of great scientific significance to ensure the water quality of the Eastern line of South-to-North Water Transfer Project. In this work, the distribution characteristics, influencing factors and releasing risk evaluation of phosphorus in the sediment in the WLFZ of Lake Nansi were investigated. The results showed that the average contents of total phosphorus (TP), inorganic phosphorus (IP), NaOH-extractable phosphorus (NaOH-P), HCl-extractable phosphorus (HCl-P) and organic phosphorus (OP) in the sediment of the WLFZ of Lake Nansi were 745.37, 510.51, 50.42, 460.09 and 234.86 mg/kg, respectively. The percentages of IP to TP in the sediments of the WLFZ of Lake Nansi were higher than those of OP to TP. The contents of different P species in the sediments of the WLFZ of Lake Nansi had great spatial heterogeneity, with TP, IP, OP, HCl-P and NaOH-P at 30.7%, 36.9%, 29.6%, 37.6% and 51.2% respectively. There was no significant difference in the contents of NaOH-P, HCl-P, IP, OP and TP in the sediments of natural wetland, arbor forest land and irrigated land. The type of sediment parent material could affect the content of OP in the sediment, but it had a negligible effect on the contents of TP, IP, NaOH-P and HCl-P in the sediment. There was a significant positive correlation between NaOH-P content and HCl-P content, and a very significant positive correlation between NaOH-P content and OP content. The pollution evaluation results showed that 5.9% of the sediment samples were slightly contaminated with phosphorus, 76.5% were moderately contaminated with phosphorus, and 17.6% were heavily contaminated with phosphorus. On the whole, the phosphorus pollution in the sediments of the 30 km² WLFZ of Lake Nansi was relatively serious. The results can provide scientific and technical support for the control of phosphorus release from sediments and the protection of water quality in the WLFZ of Lake Nansi.