Establishing precise rainfall threshold and effective forecasting system is one of the important measures to prevent debris flow disasters and reduce damage. We analyzed the rainfalls triggered by debris flows at clustering and small scales in the Wenchuan earthquake region of southwestern China during the period of 2008—2013, aiming to establish the range of rainfall intensity-duration threshold in Beichuan, Qingping, Dujiangyan and Yingxiu of Sichuan Province. The rainfall threshold in Dujiangyan was higher while that in Yingxiu was lower than in the other two regions. The threshold of critical rainfalls of clustering debris flows in Beichuan was higher than that of small-scale debris flows, but this trend could not be found in the other three regions. There was an obvious dividing line of rainfall intensity-duration between clustering and small-scale debris flows in the other three regions. Furthermore, the rainfall intensity-duration of small-scale debris flows in some regions was higher than that of clustering events. Finally, the warning and forecasting grades of debris flows were developed for the four regions based on the relationships between rainfall intensity-duration, and antecedent rainfall and real-time rainfall, which would be helpful for warning and forecasting of local debris flows.