Abstract: The dispersibility as well as seepage distortion and salt transference under long-term seepage condition of soil samples obtained from the clay core wall in Xijiao and Sanping reservoirs were studied by physical and chemical properties of soil and composition of clay mineral and the tests for identification of dispersive clay soils which included double-hydrometer test,pinhole test,crumb test,tests to defermine salt content of pore water and percentage of exchangeable sodium.It was shownd that the soil samples from Sanping reservoir were non-dispersive clays but the soil samples from Xijiao reservoir were of transitional clay.The effect water of used in constructon on dispersive clay was very little,but the effect of scouring water was obvious.Under long-term seepage condition the soil could be improved and became non-dispersive because the sodium-ion in soil could be moved away and the calcium-ion in river water could enter into the soil.The results indicate that the absolute and relative contents of sodium-ion and calcium-magnesium-ion in soil are important to identify dispersive clay soils,while the content of montmorillonoid is one of key factors and the river water affects the dispersibility of clay greatly.