The unstable slope is located in the cantonal of Chongqing City, where large foundation excavation will be carried out in the influenced scope. Analysis on stability must be taken for construction safety. On the basis of previous studies, combined with field investigation, the geological environment of this study area is summarized. By analyzing the measured monitoring data, the displacement characteristics of the monitoring point is studied. To know the deformation of unstable slopes, FLAC-3D numerical simulation is adopted to obtain soil parameters by the comparison of monitoring curves and numerical results. Finally, based on limit equilibrium theory and strength reduction method, the evaluation results of stability are studied for natural state of unstable slopes and excavation.