In this study, the characteristics and bioavailability of phosphorus fractions in surface sediments of Ganjiang River were analyzed by chemical sequential extraction method. The risk of sediment phosphorus release was explored by isothermal adsorption experiments. The results showed that total phosphorus (TP) content in the surface sediments of Nanchang section of Ganjiang River had a range from 235.21 to 702.24 mg/kg, with an average of 522.93 mg/kg, implying a high degree of spatial heterogeneity. In all sampling sites, the occluded phosphorus (Oc-P) was the principal storage form of inorganic phosphorus (IP). The inorganic phosphorus content of each form was characterized as Oc-P>Iron-bound phosphorus (Fe-P)>detrital calcium phosphate (De-P)>authigenic calcium phosphate (ACa-P)>exchangeable phosphorus (Ex-P)>aluminum-bound phosphorus (Al-P). Organic phosphorus (OP) was mainly stored in the form of residual organic phosphorus (Res-P_o). According to the division of activity, it was ordered as follows: inactive organic phosphorus (NOP)>medium-active organic phosphorus (MLOP)>active organic phosphorus (LOP). The content of bioactive phosphorus (BAP) had a range from 61.59 to 218.27 mg/kg, with an average of 145.54 mg/kg, accounting for 27.07% of TP content. The total amount of BAP and its proportion to TP were relatively low, indicating a low risk of internal phosphorus release from the sediments. The content of Fe-P in BAP accounted for 56.72%, showing that the potential risk of phosphorus release from sediments mainly came from Fe-P. There were significant correlations between TP, Fe-P, and De-P, indicating that external inputs may be the primary source of phosphorus in the sediments of Ganjiang River. During the sampling period, the equilibrium phosphorus concentration (EPC₀) of the sediments was higher than that of the overlying water's dissolved reactive phosphorus (SRP). The values of the degree of phosphorus saturation (DPS) were lower than the critical value of 25% for the large amount of phosphorus loss from the sediments, indicating that although the sediments acted as "phosphorus source" for the overlying water at this stage, the possibility of large amounts of phosphorus release was not high. Therefore, the risk of eutrophication of Ganjiang River caused by sediment phosphorus release is low, implying that more attention should be paid to external inputs to Ganjiang River water body. This study can potentially provide data support and theoretical basis for the water management of Ganjiang River.