The Wudongde dam is located in the mountainous region of southwestern China. Debris flows form a serious threat to the safety of this site and appropriate hazard assessments need to be carried out. However, the results of such assessments may be erroneous, especially when certain influencing factors are not properly considered. In this study, a stepwise discriminant analysis is used to investigate which factors influence debris-flow occurrence. A total of 27 debris flows, identified through field investigation, are analysed and this resulted in the selection of 17 influence factors. By employing a stepwise discriminant analysis it is possible to reduce these to just five major influencing factors, thus simplifying the calculation of debris-flow hazard assessment. Extension theory has been used to further identify the role of the influencing factors in debris-flow hazard assessment. Thus, reasonable and objective results of hazard degree assessment are expected. Once the criteria for debris-flow hazard assessment are established, the hazard degree of each debris flow is determined. Of the 27 debris flows analysed in this study, four are categorized as ‘extremely hazardous’, 10 as ‘very hazardous’, 10 as ‘moderately hazardous’, and three as ‘slightly hazardous’. The results are consistent with current expert knowledge of these debris flows, thereby providing an indication that the methods used in this study adequately replicate the expert assessments.