Abstract: The reactivation of ancient landslides is commonly driven by the combined effects of anthropogenic disturbances and rainfall, presenting significant risks to engineering safety. Effective selection and evaluation of mitigation measures are therefore critical for ensuring safe operation of the project. This study investigated the Chaotian ancient landslide through an integrated approach combining geological survey, on-site macroscopic inspection, and three years of deformation monitoring to deeply analyze the deformation and failure characteristics of the ancient landslide. FLAC3D was used to quantitatively reveal the control effect of excavation and rainfall on the resurrection of the ancient landslide, as well as to evaluate the effect of supporting measures. The characteristics of the early resurrection deformation of the ancient landslide are mainly characterized by the cracking and collapse near the excavation surface, and the gradual retreat along the longitudinal “traction type” in time and space, forming the No.Ⅰ and No.Ⅱ potential secondary sliding bodies in the area. Excavation-induced high and steep free faces are identified as the primary triggering factor, while rainfall acts as a secondary driver that accelerates deformation. Insufficient embedment depth of the initial support structures contributed to secondary reactivation. The reinforcement support effectively controls the deformation of the ancient landslide, and there was an obvious inflection point in the pile deformation curve. Compared with the initial support, the stability under natural and rainstorm conditions increased by 0.09 and 0.17, respectively. The combined effect of excavation and rainfall has a controlling effect on the resurrection of ancient landslides. The embedded depth of anti-slide pile end is directly related to the effectiveness of prevention and control measures.