Abstract: The semipermeable membrane behavior of clay materials can significantly delay contaminant diffusion, which is of great importance to evaluating the service performance of barriers in various landfills effectively. Using the self-made rigid wall test device, a series of chemico-osmotic tests on the kaolin-bentonite (K-B) mixtures with bentonite content (5%~60%) are carried out, and the variation trends of chemico-osmotic efficiency coefficients of K-B specimens are determined. Meanwhile, the influence mechanism of membrane effect behavior is revealed through the scanning electron microscope. The results show that the change process of chemico-osmotic efficiency coefficient can be divided into three stages, slow increase (< 30%), sharp increase (30%~40%) and steady condition (> 40%). When the content of bentonite increases from 5% to 60%, the corresponding chemico-osmotic efficiency coefficient increases from 0.002 to 0.197. In addition, the increase of bentonite content causes the pore structure change of K-B specimens, the number of large holes decreases, and the number of small holes gradually increases. Furthermore, the aperture perimeter of holes increases, indicating that the variation of pore number and aperture perimeter is the internal reason for leading to the change of chemico-osmotic membrane behavior with bentonite content.