Abstract: The Niqieka landslide is a typical ancient landslide in a deeply incised river valley on the Qinghai–Tibet Plateau. In early August 2024, continuous heavy rainfall led to the expansion of surface cracks in the landslide deposit and signs of reactivation, which seriously threatened the safety of Moka Village. To scientifically elucidate the formation mechanism and clarify the influence of tunnel excavation, an integrated Space–Air–Ground–Water investigation system was established. Combined with particle flow discrete element method and finite element numerical simulation of tunnel excavation responses, the geological evolution process of the landslide and the response characteristics of tunnel engineering were analyzed. The results show that the Niqieka landslide presents a typical "toppling-tensile failure" geomechanical model and is currently in a shallow creeping stage induced by rainfall. The discrete element simulation reveals that the attenuation of shear strength of rock and soil mass caused by continuous rainfall is the fundamental cause of the deformation, which is mainly concentrated in the shallow surface of the middle and front parts of the slope, without forming a deep and fully connected sliding surface. Compared with typical landslide–tunnel "interaction" disaster cases and combined with finite element numerical simulation, the expansion radius of the plastic zone caused by the excavation of the Niqieka tunnel is only 10−15 m, while the vertical distance from the tunnel crown to the landslide base interface is up to 70 m. The substantial burial depth of the tunnel effectively blocks the upward transmission of stress, confirming that there is no direct correlation between the Niqieka tunnel and landslide reactivation. Based on the integrated Space–Air–Ground–Water investigation and numerical inversion, this study realizes the identification of landslide formation mechanism and scientific definition of engineering influence under complex plateau canyon environments, providing a theoretical basis and reference for risk prevention and control of similar geological hazards in plateau canyon areas.