Debris flow is a very common natural hazard that causes a large number of casualties and property loss worldwide. This thesis is focusing on the material property and trying to develop a competent model for numerical simulations of debris flow. In the constitutive modelling, debris materials are normally simplified as a mixture of solid spherical particle and viscous fluid and treated as continuum. A competent constitutive model is required to capture the transition between the solid-like and fluid-like behaviors. In this thesis, we combine the constitutive theories of statics and the dynamic theories to develop a unified and multi-scale constitutive model for debris materials. A framework which consists of a static portion for the frictional behavior and a dynamic portion for the viscous behavior is proposed. Bagnold's constitutive theory is slightly modified and employed as the dynamic portion. A concrete constitutive model is obtained by using a hypoplastic model as the static portion. In the simulations of annular shear tests, partial and full liquefaction are well predicted by the hypoplastic model. The unified model is then implemented in a Smoothed Particle Hydrodynamics (SPH) code and verified by simulating some boundary value problems of granular flows. In the case of granular flow down an inclined plane, steady dense granular flow is observed over a range of inclinations, which is consistent with the theoretical analysis. For the granular pile collapse and the granular flow in the rotating drum, the numerical results show wealth of various behaviors, i.e. quasi-static motion, shear band, flow initiation, fully developed granular flow and granular deposition. Since some aspects, such as hydro-mechanical coupling and particle segregation, need further investigation, applying the unified model to the numerical simulation of debris flow in nature is still an interesting challenge.