<p>The reactivation of ancient landslides has garnered significant research attention in the context of major engineering projects in recent years. Investigation into the triggering mechanisms and deformation behavior of the Hongzhai landslide in Qinglong County, Guizhou Province, reveals that it represents a case of ancient landslide reactivation with a relatively complex developmental history. This landslide is characterized by the progressive deepening of the sliding surface and multiple reactivation events along the pre-existing slip zone. Survey data indicate that the Hongzhai landslide has a volume of approximately 6.25 million cubic meters and is classified as a deep-seated large-scale landslide. It can be divided into four distinct zones based on deformation features, failure patterns, and stress transmission directions. Reactivation was facilitated by the combined effects of steep terrain, complex rock mass structures, weak engineering rock units, and persistent surface water infiltration. Historical analysis suggests that the ancient landslide underwent three major deformation–failure events, with the slip zone progressively deepening over time. InSAR-based surface deformation monitoring indicates that most displacement occurred prior to the major failure event, with only minor movement detected afterward. Numerical simulations using MatDEM show that the frontal and middle sections experienced greater movement than the rear section, with a maximum displacement of 14&#xa0;m at the front, consistent with field observations. The Hongzhai ancient landslide serves as a typical example of multi-phase reactivation of ancient landslides. Its deformation evolution offers valuable insights for the analysis of similar landslide hazards.</p>