In recent years, debris flows induced by rainstorms have caused severe casualties and property losses in the Wenchuan earthquake region, Southwest China. The different rainfall patterns point to a composite rainfall threshold for the occurrence of debris flows. The rainfall intensity and antecedent rainfall of debris flow events were analyzed after the Wenchuan earthquake. The results show that the rainfall pattern in time can be classified into three types: a short-term rainfall pattern, a medium persistent rainfall pattern, and a long-term intermittent rainfall pattern. The main differences between the three patterns of a single precipitation event are rainfall intensity, duration, and cumulative rainfall, which can result in different infiltration, saturation, and triggering mechanisms of debris flows. The short-term rainfall pattern can be associated with the highest rainfall intensity and the least cumulative rainfall. The medium persistent rainfall pattern is related to intermediate rainfall intensity and the medium cumulative rainfall, and a long-term intermittent rainfall pattern can be related to the lowest rainfall intensity and the longest rainfall duration. At last, this paper sets up the patterns of rainfall-threshold for debris-flow occurrence. According to the critical initiation, conditions of the debris flow in the five watersheds can be predicted by the intensity-duration curve (ID curve) under different rainfall patterns.