Water level fluctuation in reservoir intensifies the water-soil interaction between river water and loose soil of landslide deposit, and thereby profoundly affecting the engineering properties of surface rock-soil. The impact of river water on the engineering property of clayey soil at the reservoir bank is studied from the perspective of water-soil interaction. Contrast tests including X-ray diffraction, Zeta electric potential, exchangeable cation types, shear test for consolidated samples, and SEM are performed to study the change of components, microstructure and shear strength of clayey soil from landslide before and after the interaction. It is concluded that Zeta electric potential at the surface of soil particle decreases under the combined effect of hydration and cation absorption. The thickness of absorbed water of soil grains decreases, and the microstructure and shear strength of remoulded soil are somewhat varied as the cations absorb divalent cations to soil grains. Moreover, dilatancy effect is generated due to the change of soil particle structure in the shear test of water-treated clayey soil. The mechanism of water-soil interaction affecting on the engineering properties of fine-grained soil is finally revealed.