Abstract: This paper presents a stability analysis of Sliding Mass I at riverside of Huangtupo Landslide, in which both rainfall and reservoir water level fluctuation in one hydrological year are explicitly considered. Based upon the properties of the sliding mass obtained from laboratory tests such as ring shear test, a GeoStudio numerical model is built to study the stability and deformation of Sliding Mass I at riverside. The comparison between the monitored deformation and the numerical analysis indicates that the use of the residual strength obtained from ring shear tests can yield a better prediction of the landslide deformation. The monitored deformation together with the numerical results illustrates that the deformation at the front edge of the sliding mass is mainly affected by the reservoir water level undulation; whereas, the deformation at the tailing edge of the sliding mass is mainly influenced by the rainfall. The stability analysis also indicates both rainfall and reservoir water level fluctuation could influence the stability of the sliding mass, and,the most unfavorable scenario could be under the combined influence of the rainfall and the decrease of reservoir water level. In other words, the stability of this sliding mass is relatively low in July and August each year, and in that period essential caution should be paid.