Abstract: Identifying the deformation characteristics of an ancient landslide induced by tunnel excavation is crucial for revealing the tunnel-landslide interaction mechanism. In this study, a ancient landslide reactivated by a highway tunnel excavation in a mountainous region of Southwest China was taken as a case study. Based on field investigation, the deformation characteristics of the reactivated landslide were systematically analyzed by integrating numerical simulation, Distributed Scatterer Interferometric Synthetic Aperture Radar (DS-InSAR) time-series monitoring, and in-situ surface monitoring techniques. The results indicate that: (1) Spatially, the landslide body exhibits significant zonation. The deformation is mainly concentrated in the area of Landslide Zone I above the tunnel, where surface displacement shows a decreasing trend from top to bottom, while the displacement in ZoneⅡ remains relatively stable. (2) In terms of temporal evolution, the landslide deformation undergoes stages of "slow creep - accelerated deformation - local reactivation" . Specifically, from July to October 2015, the displacement of the slope body above the right tunnel exhibits a distinct trend of accelerated deformation. (3) The results from numerical simulation, InSAR interpretation, and in-situ monitoring are highly consistent regarding deformation trends. The unloading effect caused by tunnel excavation is identified as the direct cause for the reactivation of the ancient landslide.