The restriction of sharp channel bends on debris flow movement will lead to strong erosion action on concave bank, which was an important reason for the expansion of the channels and the instability of the slopes. Most studies only focus on bed erosion but ignored the concave bank also suffer strong erosion action. In order to analyze the erosion mechanism, a series of erodible and rigid bank experiments with different curvature radii of bends were carried out. After obtaining the calculation equations for the shear force and the shear resistance of the concave bank in impact zone, we revised the commonly entrainment rate. Finally, we obtained a formula for calculating the entrainment rate of concave banks by debris flow, it can provide reference for the study of slope stability.