<p>Ancient landslides formed during long-term geological evolution are prone to structural instability and recurrent deformation. Analyzing the controlling factors and failure modes associated with the reactivation of ancient landslides is thus essential for hazard assessment and prevention. Using the Haijiaoping ancient landslide in Zunyi, China, as a case study, this research integrates field investigations, monitoring data, the SBAS-InSAR technique, and numerical simulations to examine its reactivation and failure mechanisms. The Haijiaoping landslide, with a volume of 1.5 × 10⁶ m³ and a surface area of 8.3 × 10⁴ m², is a large ancient landslide exhibiting a composite failure mode that combines localized secondary shear-out with overall translational sliding, marked by a double-slip-surface feature. During deformation, the sliding mass shows significant creep behavior with a progressive evolutionary trend. Rainfall is recognized as the primary trigger for the reactivation of the Haijiaoping landslide. Hydraulic and hydrological parameters play a significant role during the initial stage of rainfall, controlling the timing of large-scale slope deformation, whereas strength parameters act throughout the entire rainfall process and are the decisive factors determining slope failure. Furthermore, in areas with adverse topography or complex structural conditions, the extent of failure tends to be more severe even under the same degree of parameter weakening. These findings enhance understanding of the reactivation mechanisms of ancient landslides and provide practical guidance for disaster prevention and mitigation, as well as a reference for predicting the reactivation of similar landslides.</p>