<p>This study investigates the reactivation mechanisms and deformation evolution process of a colluvial landslide that occurred in 2018 in southwestern China. This landslide posed a threat to the nearby industrial park, and its reactivation was driven not only by geological conditions but more significantly by the combined effect of intense rainfall infiltration and the failure of the pre-existing drainage system at the slope toe. Through field investigation, drilling, monitoring, and numerical simulation, two interconnected cyclic reactivation mechanisms were identified: (1) a positive feedback loop involving rainfall infiltration, crack development, and seepage erosion, which progressively reduced the shear strength of the sliding zone; and (2) groundwater accumulation behind the anti-slide piles due to drainage blockage, resulting in excessive water pressure, structural degradation of the piles, and further deterioration of drainage performance. Numerical simulation successfully reproduced the landslide’s deformation behavior and stability evolution, showing close alignment with monitoring data. Based on monitoring, the deformation process was categorized into four stages: the Frontal Failure Stage, the Toe Creep Stage, the Retrogressive Propagation Stage, and the Stress Redistribution and Stabilization Stage. These stages clearly demonstrate that the effectiveness of supporting structures exhibits significant spatial transmissibility and temporal hysteresis. Accordingly, an integrated “drainage-supporting coupling” strategy is proposed, which advocates for an organically integrated system where drainage measures and supporting structures function synergistically with spatiotemporal coordination. This approach provides an effective and transferable theoretical framework for the mitigation of similar reactivated landslides.</p>