Abstract: The Wenchuan area is a frequent area of debris flow disasters, and there is a narrow and steep debris flow. The narrow and steep debris flow has the characteristics of large longitudinal slope gradient, large reserves of provenance and large scale of rushing out. Therefore, it is necessary to pay attention to the prevention and control of debris flow in the disaster area. In this paper, the well-balanced numerical method is used to study the narrow and steep debris flow in Wenchuan area. The flow process and flow depth of each burst frequency are studied by taking the Wa Yaogou in MianChi Town, Wenchuan County as an example. The simulation results were interpreted from the topography and fluid energy directions. The well-balanced numerical method can well handle the debris flow migration problem of complex terrain, and more accurate simulation results can be obtained for the narrow and steep debris flow. According to the simulation results, the solid matter of the debris flow with a 2%burst frequency is 1.87×104 m³,the maximum punching length is 185 m, the stacking area of the stacking fan is 2.88×104 m², and the reliability of the method is determined by comparison. The study of its flow process shows that the migration process of the narrow and steep debris flow is closely related to the topography of the debris flow ditch:the fluid energy gradually accumulates in the curve region, and the maximum value of the flow energy and flow velocity of the debris flow in the linear channel region is downstream. The sloping flow of the mudslide reaches a maximum at a relatively gentle slope, and finally rushes out of the gully. Based on the above conclusions, it provides working methods and reasonable engineering suggestions for the treatment and prevention of narrow and steep debris flow disasters in WenChuan.