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The excavation of the tunnel on the expressway in northwest Yunnan province induced landslide and a series problem such as ground surface cracks of the slope, sliding of the slope, and cracks in the tunnel lining. This research aims to reveal the interaction relationship between the tunnel and the landslide from the prospective of field monitoring and numerical simulation. Firstly, the engineering geological conditions of the slope where the tunnel was located were obtained by field investigation. The “landslide traction segment-tunnel longitudinal tensile failure” mode was put forward based on the spatial relationship between the tunnel and the landslide. Secondly, field monitoring methods were adopted to monitor the surface displacement of the slope, the deep-seated displacement of the landslide, and the propagation of cracks in the tunnel lining. Finally, three-dimensional numerical models were established to investigate the stability of the slope and the tunnel under natural conditions, tunnel excavation conditions, and rainfall conditions. The field investigation results, field monitoring results, and numerical simulation results illustrated that: (1) The tunnel traversed the traction segment of the landslide body in parallel, and tensile failure or shear dislocation failure would occur at different stages of the interaction between the tunnel and the landslide. (2) Two sliding layers were discovered in the landslide, the shallow creep sliding layer and the deep creep sliding layer, which corresponded to the tensile failure and shear dislocation failure modes proposed in the “landslide traction segment-tunnel longitudinal tensile failure” mode, respectively. (3) The slope was in an unstable state under natural conditions. The tunnel excavation disrupted the initial stress equilibrium of the slope, resulting in stress release of the surrounding rock mass. Both excavation and rainfall would exacerbate the deformation of the landslide and the tunnel. Eventually, control measures based on the control grouting technology of the steel floral tubes were suggested to counter with landslide-tunnel deformation problems.