Most debris flows are formed by slope failures, performing as landslides, collapses, and so on, which not only supply solid materials in debris flows, but also control the formation approach and magnitude variety. Field artificial rainfall experiments on slopes in debris flow source region in Niujuan Gully in Wenchuan County and Wei- jia Gully in Beichuan County show that the slope failure is actually a random, discontinues process with fluctuation in magnitude. The collapse interval exponentially decreases, while collapse magnitude exponentially increases with rainfall intensity. The relationship between collapse number, average collapse magnitude and the rainfall intensity can be described as N = CN exp(kN IR) and M = CMexp (kM IR) , thus the total collapse amount can be expressed as V = NM = CNCu exp ( (kN + kM)IR). The parameters in the equation is various due to the viriety of grain composition of the soils on slopes. This study shows the exponential amplification effect of rainfall on slope failure intensity, which can be regarded as the formation mechanism of giant debris flow in rain storms.