The phenomenon of debris flow is intermediate between mass movement and solid transport. Flows can be sudden, severe and destructive. Understanding debris flow erosion processes is the key to providing geomorphic explanations, but progress has been limited because the physical-mechanical properties, movement laws and erosion characteristics are different from those of sediment-laden flow. Using infinite slope theory, this research examines the process and mechanism of downcutting erosion over a moveable bed in a viscous debris flow gully. It focuses specifically on the scour depth and the critical slope for viscous debris flow,and formulas for both calculations are presented.Both scour depth and the critical conditions of downcutting erosion are related to debris flow properties(sand volume concentration and flow depth) and gully properties(longitudinal slope,viscous and internal friction angle of gully materials,and coefficient of kinetic friction). In addition, a series of flume experiments was carried out to characterize the scouring process of debris flows with different properties. The calculated values agreed well with the experimental data. These theoretical formulas are reasonable, and using infinite slope theory to analyze down cutting erosion from viscous debris flow is feasible. The phenomenon of debris flow is intermediate between mass movement and solid transport. Flows can be sudden, severe and destructive. Understanding debris flow erosion processes is the key to providing geomorphic explanations, but progress has been limited because the physical-mechanical properties, movement laws Using erosion theory are different from those of sediment-laden flow. It focuses specifically on the depth of scour depth and the critical slope for viscous debris flow, and formulas for both calculations are presented. Both scour depth and the critical conditions of downcutting erosion are related to debris flow properties (sand volume concentration and flow depth) and gully properties (longitudinal slope, viscous and internal friction angle of gully materials, and coefficient of kinetic friction). In addition, a series of flume expe rized was to characterize the scouring process of debris flows with different properties. These calculated formulas agreed well with the experimental data. These calculated formulas are reasonable, and using infinite slope theory to analyze down cutting erosion from viscous debris flow is feasible.