A mathematical model of the process of debris-flow deposition has been constructed using the principles of mass conservation and momentum conservation with the deep-gully debris flows in Dongchuan City, Yunnan Province as the object of study. The authors attempt to seek the solution using the finite difference method and simulate the dynamic process of deposition of debris flows in regard to the actual characteristics of their movement. On the basis of that result, the authors have analyzed and evaluated the risk scope and extent of deep-gully debris-flow disasters and further analyze and estimate the possible loss caused by the disaster at 2% fren-quency in combination with separate statistical analyses and estimates of the values of various kinds of assets in the risk area. An effectiveness analysis of disaster-prevention engineering works and their costs has been carried out and related countermeasures and suggestions have been put forward.