Abstract: Right now exploring the ways and means in preventing and treating the debris flows in the cities and towns of the mountainous regions is of primary importance for the construction of "Four Modernizations". This paper bases on the findings of the investigations on spot to describe briefly the natural conditions, the man-made factors of the formation and the development of the in situ debris flows. We also analyse the causes of the debris flow damages to the district city of Hanyuan and put forward the principle and the measures in preventing and treating the debris flows in that city.