Abstract: The effect of groundwater caused by heavy rainfall plays an important role in outbreak of gently inclined rocky landslide in red stratum region in east Sichuan. Taking a typical gently inclined rocky slope for example, we analyzed the groundwater seepage law through field investigation and numerical simulation, and studied its deformation and failure mechanisms from groundwater physical and mechanical effects. The results show that: groundwater in layered and blocky rock mass slope mainly migrates through primary bedding and joint cracks; pore water pressure is mainly concentrated in the rear edge of the slope, which gradually decreases near the leading edge. The landslide is caused by the combined effects of hydrostatic water pressure and physical softening and argillization; it deforms under the effect of horizontal thrust, uplifting pressure and shear resistance decreasing of sliding zone soil due to rainfall infiltration. The formation and evolution process of the landslide could be divided into 3 stages:  formation and penetration of structure plane； softening and weakening of mud rock interlayer； deformation triggered by heavy rainfall.