[Objective] Studying the deformation rule and failure mechanism of Huangnibadengkan slope in order to achieve accurate prediction of landslide and provide reference for similar type of landslides research. [Methods] Through the analysis of its macroscopic deformation characteristics and based on professional monitoring data, the deformation rule and failure mechanism of this slope was studied. Meanwhile, slide mechanism mainly caused by water level falling is analyzed using the numerical calculation. [Results] Serious deformation happened in slide mass Ⅰ of Huangnibadengkan slope, which experienced accelerating deformation for three times, occurred in April to June in 2007, 2009 and 2012, respectively. The effects that reservoir water drawdown makes on this landslide are considered as two aspects: unloading on slope surface and internal hydrodynamic pressure. That means, on one hand, the horizontally slope-inward hydrostatic pressure disappears, as a result, the resistance force will decrease; on another hand, the underground water level drops triggered by reservoir water drawdown, will result in outward deformation of the landslide pushed byhydrodynamic pressure. [Conclusion] The Huangnibadengkan landslide is in an unstable state at present, and falling of reservoir water level and continuous heavy rain are proved to be the main factors that lead to the deformation of this landslide.