After the Wenchuan earthquake on May 12th, co-seismic landslides and fractured slopes were more susceptible to rainfall-induced shallow mass re-mobilization and post-earthquake disasters were gained widespread significance for the disaster mitigation. However, despite the rainfall thresholds, the hydrological parameters of rainfall induced mass re-mobilization in natural environment of Wenchuan earthquake regions is not well understood and widely used for disaster early warning. In this study, shallow rainfall triggered slope failures under partially saturated conditions in the hollows of the gully was proved by instrumental evidence of in situ experimental tests in a natural co-seismic landslide for simulating the rainfall triggered erosion process of shallow failures in debris flow catchment. In addition, the results revealed the transient process and unsaturated condition for mass movement in response to rainfall, and demonstrated the importance of hydrological parameters includes soil matrix suction and moisture content for shallow slope failure in the hollows, and the stability analysis suggested a hydro-mechanical thresholds including water contents and matrix suction based on the mechanism of slope failure for early warning of the mass-remobilization in hollows of debris flows. These findings were expecting for contribution effectively on improvement of early warning accuracy for rainfall induced shallow landslides and debris flows in earthquake hit region.