Abstract: The debris flow happened on 8th august 2010 in Zhouqu county is a disaster which caused the biggest damage in Chinese recorded history.It is extremely meaningful to faithfully record the features,forming reasons and triggering factors of this debris flow,analysis the development and propose the risk mitigation measures for the reconstructions guidance and further study of this disaster.This paper takes the debris flow at Sanyanyu gulley as an example.The debris flow features were described according to the zoning and deposition characteristics,unit weight,velocity and discharge of the debris flow.The forming reasons were analyzed based on the topography,solid material properties,and the extremely rainfall conditions.The construction blockage of the debris flow outlet,disadvantageous human activities,and environmental degradation because of deforestation were recognized as the main reasons for this huge disaster.The potential debris flow at Sanyanyu gulley is developing.It is probable for the debris flow to occur again,and the frequency and scope will increase.A comprehensive risk mitigation measure incorporating local let-by,engineering treatment,monitoring,warning and public education was put forwarded.