【Objective】Eutrophication of the water body is the main environmental problem the operation of the Three Gorge Reservoir (TGR) has to face. Phosphorus in the water body is considered to be the key restrictive factor of eutrophication. During the operation of TGR, the water level in the reservoir rises and falls periodically, thus causing the formation of a hydro-fluctuation zone, where the soil gets inundated and exposed to air periodically, too. The periodical alternation of exposure amd inundation of the soil significantly affects the circulation and migration of phosphorus in the soil, as well as the P loading of the water body. Parts of the hydro-fluctuation zone are used for agricultural production during the low water level period, resulting in an excess accumulation of phosphorus adsorbed in the soil, which becomes a source of phosphorus loading into the overlying water during the flooding period, thus enhancing eutrophication of the water body. 【Method】Therefore, an in-lab incubation experiment was carried out of the purple alluvial soil and grey-brown purple soil, which are widely distributed in the Three Gorges Reservoir area to explore characteristics of the variation of extraneous phosphorous in activity and morphology in the two soils as affected by saturation degree of the nutrient. 【Result】Results show as follows:, (1) Once extraneous phosphorous entered the soils, the content of Olsen-P, which characterizes soil P availability, and soil P releasing ability declined exponentially, which may be described with the equation of C_t = ae^-kt +b, with fitting degree being as high as about 94% for both soils, and the drop was the sharpest during the first 15 days of incubation, and then leveled off; (2) Extraneous phosphorous in different soils differed in decline of availability, equilibrium concentration and release potential. It remained quite higher in the gray-brown purple soil than in the purple alluvial soil in availability and consequently in risk of leaching loss and releasing; (3) The release potential of soil phosphorus was governed by saturation degree of phosphorus in absorption. Fifty percent of Q_m (the maximum adsorption capacity) was the turning-point of Olsen-P and phosphorus equilibrium desorption in both of the soils. When the phosphorus sorption saturation degree reached over the point, availability and leaching risk of the soil phosphorus increased rapidly; (4) A significantly positive relationship was observed between Olsen-P and phosphorus release at the p<0.01 level, and could be well fitted with a linear equation. It is, therefore, feasible to characterize the potential of soil phosphorus release with Olsen-P content; (5) Once extraneous phosphorus entered the soil, about 50%~60% of it transformed into Ca₂-P and Ca₈-P, which were higher in availability, while about 30% turned into Al-P and Fe-P, and O-P (Occluded phosphorus )and Ca₁₀-P did not change much in content; and (6) Ca₂-P was the main form of IP that determined Olsen-P content and soil phosphorus desorption capacity, and the effects on the two were positive and direct. 【Conclusion】Fifty percent of Q_m (the maximum adsorption capacity) was the turning-point of Olsen-P and phosphorus equilibrium desorption in the purple alluvial soil and grey-brown purple soil, The release ability of phosphorus can be predicted by Olsen-P content.