<p>The reactivation of ancient landslides triggered by engineering activities constitutes a major challenge for hazard mitigation in mountainous urban regions. This study conducted a comprehensive investigation of the Hongyan landslide located in Guiyang city, Guizhou Province, China, which experienced reactivation in 2018 subsequent to staged slope excavations. An integrated approach incorporating field surveys, borehole drilling, inclinometer monitoring, and FLAC3D numerical simulations was utilized to characterize the deformation behavior and elucidate the failure mechanism. The findings indicate that the formation of the Hongyan landslide was controlled by regional lithology, topographic configuration, geological structures, and anthropogenic disturbances. The presence of clay interlayers within the gravelly soil, along with the preserved stratified structure, enhances the susceptibility to sliding. The free face of the slope creates favorable conditions for subsequent deformation of the sliding mass. Furthermore, as a component of an ancient landslide deposit, the sliding mass retains the primary stratified structure of the source formation. Slope excavation activities diminish shear resistance, alter free face conditions, and directly instigate landslide reactivation, thereby constituting the decisive factor among multiple contributing elements. During deformation, the sliding mass demonstrates a counterclockwise deflection, attributable to variations in free face conditions. The reactivation of the Hongyan landslide entails a complex interplay of multiple predisposing factors, leading to a particularly intricate failure mechanism.</p>