In the landslide-induced debris flow problem, the soil slope experiences three stages: slope instability, debris flow and deposition. This study develops a three-dimensional smoothed particle hydrodynamics model which adopts elastoplastic viscous constitutive relation to simulate this complex process. The Drucker-Prager model with nonassociated plastic flow rule is implemented for slope instability and deposition and constant viscosity is used for propagation stage. The model is validated with laboratory dry-granular dam-break experiments. Good agreements are found between the simulated results and the laboratory data on both the shape evolution and the velocity field. Then, we use the model to study the Yigong avalanche after detailed discussion of model parameters, the time of duration, deposited scope and estimated velocity essentially agree with site survey. Thus, the model is able to simulate instability and flow of dense granular materials at both experimental and field scale, and it could be a powerful tool for the landslide-induced debris flow study.