The shear strength parameters property of rock-soil aggregates in embankment slope of reservoir,that is,the relationship between cohesion and gravel content,between friction angle and gravel content,and the relationship between cohesion and water content,between friction angle and water content,is studied based on the direct shear test results,the shear strength change law of the rock-soil aggregates is given,and the unsaturated shear strength formulation of rock-soil aggregates that could consider suction and saturation degree influence is put forward in this paper,through which the sliding or failure physical mechanism of this type of slope under the condition of rainfall infiltration is studied. Also the 3D unsteady saturated-unsaturated seepage field and its FEM resolving mode are established based on the analysis of the slope rainfall infiltration process. Case study with this method indicates that the minimum safety factor of the accumulated rock-soil aggregates dose not arrive at the moment of rainfall cessation,but appears several hours after the rainfall cessation,this phenomenon is in accordance with the practical slope engineering’s failure process and could explain appropriately the slope failure caused by rainfall infiltration. Research results in this paper have an important reference value for the research on stability of the accumulated rock-soil aggregates in embankment slope of reservoir,and can enrich the stability analysis method and relevant theory of reservoir embankment slope. The shear strength parameters property of rock-soil aggregates in embankment slope of reservoir, that is, the relationship between cohesion and gravel content, between friction angle and gravel content, and the relationship between cohesion and water content, between friction angle and water content, is studied based on the direct shear test results, the shear strength change law of the rock-soil aggregates is given, and the unsaturated shear strength formulation of rock-soil aggregates that could consider suction and saturation degree influence is put forward in this paper, through the the sliding or failure physical mechanism of this type of slope under the condition of rainfall infiltration is studied. Also the 3D unsteady saturated-unsaturated seepage field and its FEM resolving mode are established based on the analysis of the slope rainfall infiltration process. Case study with this method indicates that the minimum safety factor of the accumulated rock-soil aggregates dose not arrive a t the moment of rainfall cessation, but also several hours after the rainfall cessation, this phenomenon is in accordance with the practical slope engineering’s failure process and could seem decent the slope failure caused by rainfall infiltration. Research results in this paper have an important reference value for the research on stability of the rock-soil aggregates in embankment slope of reservoir, and can enrich the stability analysis method and relevant theory of reservoir embankment slope.