In this paper, the erosion and entrainment behavior of debris flow is investigated by three-dimensional finite element analysis based on Coupled Eulerian-Lagrangian (CEL) method. Special attention is given to the strain softening model in capturing the soil strength loss under large strain. The appropriate case studies needed for verifying the three-dimensional (3D) numerical methodology are presented. It is found that the numerical method shows good agreement with the general trend observed by the experimental tests and thus, represents a realistic phenomenon of debris flow. It is found from that a rigorous numerical analysis can provide a potential influence of debris flow and guideline on countermeasures.