The eight events of debris flow happened in Wenjiagou catchment in the Wenchuan earthquake area, Sichuan, are different from either the general ravine type or the slope slipping type of debris flow, they are resulted from rainfall induced infiltrating deformation failure and erosion of the earthquake—landslide mass. The total volume of the eight events of debris flow from the Wenjiagou earthquake—landslide mass is about 180×104m3, in which, the maximum one with the volume of 115×104m3 happened in Aug. 13, 2010. The mechanism of the Wenjiagou debris flows is that of rainfall induced seepage and piping, temporary dam water, bursting mass sliding of landslide mass in the early valley forming stage, and headward erosion, washing and digging away, side erosion and collapse, mixing flow (similarly to the stirring machine action) in the late valley enlarging stage. The two events of debris flow happened in June 21, 2008, and July 31, 2010, are resulted from the mechanism of the early valley forming stage, and the others are resulted from the mechanism of the late valley enlarging stage. The inherent mechanism of the slope failure and induced debris flow is that the capacity of rainfall infiltration is not enough to cause groundwater stagnant and water level increasing in losing landslide mass. When temporary dam water is increasing up to the horizontal level, the hydrodynamic pressure is up to the maximum value, then the slope stability decreases to the lowest, and finally, the critical condition of failure slide will be appeared in the surface of landslide mass.