Abstract: Based on the investigation of engineering geological hazard of an unstable slope along Xiangxi avenue, the topography and geomorphology, formation lithology, rock mass structure, original design scheme and construction process were studied in detail so as to analyze the slope deformation mechanism. A geological FLAC^3D model of the unstable slope was described by engineering geological profile, and then the process of retrogressive landslide due to top-down construction of lattice retaining wall was analyzed in the paper. The slope evolvement sketch of mechanism from deformation to failure was described. The results show that the slope deformation was controlled by engineering geological conditions and the change of mechanics equilibrium condition due to slope excavation, caused slope deformation and instability; because of overbreak, the damage of lower supporting rock under structural deadweight led to the lattice retaining wall subsidence, slope deformation, and retrogressive landslide; the retrogressive sliding may cause the distortion and shearing slide plane in the slope, and when the sliding plane cut through, the slope would fall; slope reinforcement based on the analysis of deformation mechanism focuses on supporting the lattice retaining wall and controlling the distortion of potential shearing exit; the monitored deformation curve indicates that the slope is stable after urgent treatment.