Abstract: Natural adsorption of phosphorus and the phosphorus force adsorption of different grain size on three samples of sedimentstaken form Xiangxi river Bay Three Gorges Reservoir were determined in laboratory.The results indicated that the totalphosphorus (TP)concentration of overlying water was significantly correlated to the TP concentration of sediment, the TP concentrationof sediment influenced the water quality of the overlying water greatly.The critical equilibrium concentration of three sampleswere C1:0.188mg/L,C2:0.147mg/L and C3:0.188mg/L, respectively.And the saturated adsorption capacities were ΔQ1:268.16mg/kg, ΔQ2:57.96mg/kg, ΔQ3:267.27mg/kg ;the two items were consistent.The influence of grain size on phosphorusadsorption was significantly related to the physico-chemical properties of sediment.The adsorption capacities of three unfilteredsamples were 1.5mg/g, 0.8mg/g and 1.4mg/g respectively.The result also indicated that the saturated adsorption capacitiesdidn' t change according to the largest adsorption capacities.The phosphorus in sediment of xiangxi river was a source to overlyingwater until the environment condition changed.If the saturated adsorption capacities, largest adsorption capacities and criticalequilibrium concentration can be correlated mathematically, the Department of the Enviroment will have a scientific basis to administerindustries near the Xiangxi River.