Fuzzy mathematics is introduced to the basis of the Extenics theory to form an Extenics analysis method based on fuzzy mathematics. Take an example of geological disaster of debris flow occurred during the rainy season in Qingyang of Huinan County, Extenics theory based on fuzzy mathematics was adopted to comprehensively analyze the risk degree of debris flow gully. The risk degree of debris flow was evaluated by the comprehensive factors, and the specific control scheme was put forward.