Debris flow is a kind of natural disaster extremely destructive to life and property, have such characteristics as outbreak in a short time, strongly damaging and simultaneous occurrence, etc. so, it is necessary to research debris flow hazard assessment. This article builds a risk assessment model of debris flow not only base on many concept of the Rough set theory such as upper and lower approximation, reduction of values, dependency graph of knowledge, but also base on case based reasoning, similarity degree of similarity system. In this hazard assessment model, basing rough set theory, the dada comes from statistical data of thirty-seven debris flow ditch of Yunnan. Proposing calculation method of synthesized weights to calculate the weight of impact factors, then sure the risk degree. By comparing the simulation results with those obtained consequence from the measured value, the risk assessment model is proved to be credible.