<p>Precipitation is a critical triggering factor in the phenomenon of slope failures in loose soil, attributable not only to its seepage impact but also to the erosive potential of runoff. In this paper, the shallow failure mechanism of slopes is analyzed through flume tests, and it is found that, under the combined action of soil seepage and surface runoff, loose slopes undergo a shallow traction-type fluidized instability failure. Based on the flume observations, the coupling effect of soil seepage and surface runoff on slope failures is analyzed by establishing a hydro-mechanical coupling model. The Navier-Stokes and Brinkman-extended Darcy equations are employed to describe the runoff along the slope surface and the seepage within the soil, respectively. The flow velocities of seepage within the soil and runoff on the surface are derived by adopting a continuous boundary condition at the liquid-solid interface. A modified slope stability equation is derived by introducing the drag force exerted by runoff, which is affected by rainfall intensity. A sensitivity analysis of key parameters (i.e., rainfall intensity, runoff coefficient, slope angle, cohesion, and internal friction angle) is also discussed. Additionally, the proposed calculation model is validated by the Wachangwan landslide.</p>