The Zhangmu landslide is a large ancient landslide located on the China-Nepal border. Many local slope failure events occurred at this site in the past, posing a direct threat to people and property. One hundred borehole logs (total drilling length of almost 5500 m) were collected to investigate the inner structure of the landslide. Electron spin resonance (ESR) dating and width measurement of annual tree rings were used to determine the reactivation time of different parts of the landslide. A bromide tracer experiment and measurements of the distribution of borehole groundwater were combined to analyze the flow field in the study area. The alluvial stratum in the study area indicates a long-term stable deposition environment. Based on the distribution of the alluvium stratum, we identified the area least prone to landslide damage. Both the surface and underground deformation characteristics were analyzed based on displacement monitoring data from 2006, 2011, and 2013. Combining the monitoring result with the lithology and precipitation, we found that landslide reactivation is closely related to precipitation, suggesting that the reactivation began in the old shear zone.