Two giant debris flows occurred in Bayi gully in Dujiangyan County,respectively on August 13and 18,2010,affecting seriously local people’s daily production and life.Based on field investigation data obtained from Bayi gully,the causes and the dynamic characteristics of the debris flows was analyzed,and a detailed risk assessment were carried out.The study results show that the loose material source,rainfall amount and topographic conditions of Bayi gully were contributed to the initialization of debris flows.The debris flows in Bayi gully were giant in magnitude and high in risk.The study provides a reliable basis for the debris flow prevention and mitigation in Bayi gully.