Abstract. Debris flows are multiphase, gravity-driven flows consisting of randomly dispersed interacting phases. The interaction between the solid phase and liquid phase plays a significant role on debris flow motion. This paper presents a new two-phase debris flow model based on the shallow water assumption and depth-average integration. The model employs the Mohr–Coulomb plasticity for the solid stress, and the fluid stress is modeled as a Newtonian viscous stress. The interfacial momentum transfer includes viscous drag, buoyancy and interaction force between solid phase and fluid phase. We solve numerically the one-dimensional model equations by a high-resolution finite volume scheme based on a Roe-type Riemann solver. The model and the numerical method are validated by using one-dimensional dam-break problem. The influences of volume fraction on the motion of debris flow are discussed and comparison between the present model and Pitman's model is presented. Results of numerical experiments demonstrate that viscous stress of fluid phase has significant effect in the process of movement of debris flow and volume fraction of solid phase significantly affects the debris flow dynamics.