Slope instability in mountain area often causes serious accidents such as traffic jams, river diversion and significant casualties, which also brings huge economic losses and potential safety hazard. Based on the detailed geological investigation and the test analysis of the slope of the Pingxi river valley, all structural planes exposed of the slope were surveyed, also the historical forming mechanism of the slope was studied. And the research results indicate the slope is formed gradually by reconstruction in the surface and the main deformation is creep. Combined with the site survey and the basic mechanical parameters from laboratory test, the two-dimensional stability numercial modelling in different cases was carried out based on the limit equilibrium method and the strength reduction method. The minimum safety factors derived is 0.94, which can not meet the specification. As a result, a landslide reinforcement scheme was suggested.