Abstract: "8.8" extremely big debris flow in Sanyanyu belongs to typical gully debris flow with obvious forming zone,transportation zone and accumulation zone.Based on site investigation,it is found that the gully head of upstream Dayanyu and Xiaoyanyu,situating at the up streams of Zhuta gully and Qiaozi gully,is the forming and accumulation zone of flood.It is also noted that the area between the forming zone and Guanziping-Yuzhi gully is the main solid source area.Area between Dayanyu-Xiaoyanyu and Sanyanyu is dominated by transportation channel.Finally,the accumulation area is situated between the mouth of Sanyanyu and Bailong River.By analyzing the zoning charateristics of Sanyanyu basin and erosion and siltation variation of debris flow in different zones,this paper can provide the basis for the establishment of Sanyanyu debris flow monitoring and warning system.