Debris flow is a natural disaster in mountainous areas,so estimation of its criticality is substantial in regional disaster prevention and reduction.Based on system theory and the analysis of the basic situation of debris flow disaster in Miyun County,this study uses analytic hierarchy process to quantify 10 factors which affect debris flow risk degree and construct a model for debris flow risk degree assessment.In the model,the percentage of inner factors is 41.4% and the percentage of external factors,58.6%.The model is applied to assess the risk degree of local debris gullies.Result obtained has a high relativity to the real situation.The study indicates that this kind of assessment is feasible and veracious for the assessment of the debris flow risk degree.The future tasks should be the reliability research on study methods and thus provide the accurate evaluation result for users.