Abstract: The safety of core wall type rockfill dams during reservoir impounding is greatly affected by the slaking deformation of upstream shell materials. The slaking deformation of the rockfill materials is analyzed using the triaxial slaking tests, in which the specimens with natural water content are compressed to various deviatoric stress levels, and then saturated by water flow from the bottom while the stress condition remains unchanged. The results show that there is a hyperbolic relationship between the slaking axial strain and the stress level before slaking for rockfill materials. Compared with the exponential function expression for the relationship between the slaking axial strain and the stress level, the hyperbolic expression is more suitable. Plenty of test data show that the ratio between the volumetric strain increment and the axial strain increment remains constant during sample saturating. Based on the above experimental findings, a simple model is proposed for predicting the deformation of rockfill materials induced by slaking, in which, the slaking secant modulus and the slaking Poisson's ratio are expressed. Comparied with the modified Shen Zhu-jiang's slaking deformation model, the curves calculated by the proposed model are more in good agreement with the experimental data. And the research shows that the slaking deformation calculated by traditional Prandtl-Reuss flow rule is smaller than the model numerical value under the high slaking stress level.