Abstract：The national highway No.212(G212)(Lanzhou—Chongqing) is a main link to connect northwestern region to southwestern region in China. The mountains area along G212 in Longnan is one of the most famous areas for debris flow. Therefore，in order to ensure proper construction and safe operation of the G212，reason and spatial-temporal distribution characteristics of debris flow in Longnan area along G212 are considered. The research area is located in the northeastern fringe area of Qinghai—Tibet Plateau，drape strip of west Qinling. Complicated geological structure，instable faults，fragmentized rock，intensely new structural activities，mass and concentrative rainfall，asymmetric vegetation coverage and improper human activities provide necessary preconditions for debris flow disaster. Firstly，through consulting literatures and field investigation，the engineering geological environment is analyzed，which includes geological structure，stratum and lithology，engineering geological rocks，terrain and physiognomy，new structural activities，rainfall，vegetation and human activities. Then，based on the understanding of the mechanism of landslides in this area，the spatial and temporal characteristics of the debris flow distribution are studied by investigating typical debris flow gullies and linking the engineering geological environment. Debris flows mainly exist in the Bailong River，Min River(the first-level branch of Bailong River) and Beiyu River. The distribution characteristics of time are discussed from three aspects：daily period，month period and age period. Among of them，debris flow appears in July and August based on month period. The results can provide some references to the route selection and proper approaches and measures to deal with geological disasters.
