Abstract: The mechanical behaviors of the interface between sands and clay are investigated through direct shear tests on the conditions of sands with different particle size and different moisture contents by a DSJ-2 electric quadruple iso-strain direct shear apparatus. The results indicate that normal stress, particle size and moisture content of sand have important influence on shear strength. As shear stress reaches peak value in scheme 1 and 2, shear strength and shear displacement increase with normal stress increase. Whenσ_n=100 kPa, τ-δcurve of the interface in scheme 1 shows strain softening phenomenon. The shear strength of the interface of scheme 1 reduces along with the reduction of sand size. When σ_n=100 kPa, in scheme 2, shear strength of interface decreases with the increase of higher water content of sand, but whenσ_n>100 kPa, it decreases first and then increases with the increase of water content of sand. Under low normal stress in scheme 2, the sensitive interval of water content corresponding to shear strength of the interface is10%,15, and more obvious.