In the study, we carried out a 3-D analysis to assess the influence of groundwater level changes on the slope stability, conducting a series of back-numerical analysis to delineate the critical line of the shear strength of the failure surface of a landslide, and a laboratory test to determine the geo-mechanical properties of soil samples. The analysis result shows that the shear strength determined by the laboratory test was distributed below the critical line of shear strength estimated by back-analysis. Differences between driving and resisting force were also analyzed in groundwater conditions of dry and saturation. It appeared that the stress gets greater towards the slope center of the landslide, and the debris mass moves downwards. According to the analysis, the factor of safety becomes 1 with the rise of foundwater level up to -0.85 m from the slope surface, while the slope tends to stay stable during dry seasons.