Terrain modifications significantly influence landslide formation and dynamics. On December 18, 2023, a catastrophic landslide struck the Jintian Village in Gansu Province, China. While triggers such as earthquakes and irrigation are well-studied, the impact of terrain modifications, particularly the extraordinary runout and the severe damage it caused, remains less explored. This study integrates field investigations, remote sensing analysis, and numerical simulations to reconstruct the landslide’s dynamics and assess how terrain changes affect its behavior. Our findings reveal that land reclamation, which involved valley infilling to create flat areas, altered regional hydrology by raising groundwater levels. This elevation promoted soil liquefaction and increased terrain instability, heightening the susceptibility to seismic activity. Additionally, the distinctive throat-like terrain regulated the volume and flow rate of landslide material, controlling the disaster’s scale. Although check dams initially restrained the flowing mass, their eventual failure amplified the landslide’s volume and potential energy, extending its reach and worsening its impact. The results underscore the need for regions with loess, particularly those prone to earthquakes and groundwater fluctuations, to carefully evaluate the detrimental effects of terrain modifications. Furthermore, the structural integrity of check dams should be reexamined to mitigate the risk of similar disaster chains.