The breaching of a large-scale landslide dam does not only release a huge amount of water, but also incur massive soil and rock sediments by complex erosion and deposition. The sediment propagation would significantly change the topography and geological environment and affect the planning of the relevant buildings and infrastructure projects in a long term. Existing studies mainly focus on the development of the dam breach and the evolution of breaching floods. Less concern is given to the simulation of the dam materials propagation during the drastic dam breaching, which is of the major target of the present paper. A comprised model, which consists of a dam breaching model (DABA) and a debris flow model (EDDA) is developed to simulating the whole sediment propagation during dam breaching. In that model, DABA is applied to mimic the erosion of dam materials, EDDA is applied to mimic the deposition of the sediment downstream of the dam, and a model of DTE is applied to link these two models by mixing the soil and water and obtaining the hydraulic parameters of the mixed fluid. The comprised model is applied to simulate the whole breaching process of the Tangjiashan landslide dam caused by the 2008 Ms 8.0 Wenchuan earthquake. The timely water depth, flow velocity, and deposition thickness along the river are obtained from the simulation. It is found that the thickness of the soil and rock sediments gradually decreases along the river from the dam body to the Beichuan; the thickness in the vicinity of the dam can reach 40m, which is similar to the height of the residual dam; the thickness is about 15m near the Beichuan. The sediment thickness at the bend of the river is large, and the sediment is mainly distributed on the convex bank