Abstract：The debris flow is a common natural disaster in southwest mountain areas.Accurate classification of debris flow risk has a great significance for local government to prevent or mitigate the disaster.At present, the common classification method is used to select a few indicators, then to assign some values and to make relevant classifications.But some classification methods cannot flexibly choose the classification geological indexes, and some classification indexes can influence each other.This paper combines the extension theory methods with the risk classification of debris flow.It establishes the method of risk classification for debris flow based on extension theory.The method mainly selects the important parameters of risk classification indexes that can reflect and embody the feature, the scale of debris flow, the reserves of loose accumulation, debris flow frequency, drainage area, length of main ditches, the maximum relative altitude in drainage area, the average slope of main ditches, length proportion of debris supplement and maximum rainfall in 24 hours.Then based on matter element theory, extension set and dependent function calculation, the matter element model of risk classification is established.And the factor weights were determined using the minimum square method.It follows the degree of association calculations for the actual risk classification.The final risk classification can determined.Through the practical application of the risk classification based on extension theory, this method has a good applicability in the risk classification of debris flow.

Keyword：debris flow;hazard classification;extendible theory;minimum square method for weight;