Wenchuan Earthquake not only induced a large amount of collapses and landslides immediately,but also reduced the strength of rock and soil mass in drainage basin,which makes them to be eroded more easily,so that abundant solid matter is provided for the formation of debris flows.The occurrence of debris flow is frequent in the disaster area after the earthquake and great losses have been caused.Evaluation on the risk degree of debris flow ditches is a necessary condition for post-disaster reconstruction.Three important parameters,i.e.,increment of solid matter,seismic intensity,and scale of barrier dam,are selected and evaluation method on increase coefficient of debris flow risk degree after earthquake is put forward based on designed debris flow.In this method,increase of debris flow risk degree is expressed by the ratio of added volume of solid matter to gross obtruding matter in a debris flow,seismic intensity multiplying the ratio,and time(hour)needed to drain water body in barrier dam with P_50% flux of the valley.By the validation of calculation for 9 typical debris flow ditches in the disaster area,the increase of debris flow risk degree in the May 12th disaster area after earthquake can be assessed by the new method,rapidly and precisely.