Abstract: Through investigation of the characteristics of the Malanghe debris flow on the east bank of the Nujiang River in Baoshan City, Yunnan Province, and determination of the age of the diluvium by OSL, this paper holds that the debris flow gully is young and dangerous. According to the complexity and randomness of specific factors which control and affect the dangerous degree of the debris flows, the fuzzy comprehensive evaluation was adopted to evaluate the fatalness of the debris flow. In this paper, seven factors were chosen as the main relevant factors. The evaluation result is the same as that of the field investigation, and the fatalness is in the degree of moderate risk (Ⅲ). Two key points in the evaluation process are pointed out in this paper, i.e., the establishment of the weight sets and the determination of the degree of membership. The first point is achieved and optimized by minimizing the distance summation between “ideal points”; and the second is implemented through the membership function designed on the basis of the half trapezoid distribution model. Repeated experiments have confirmed that this method is workable and more practical than other methods.