Abstract: Based on the non-equilibrium continuous equation and momentum equation for water-sand mixture, considering the impact on the variation of sand volume concentration due to the change of breach slope and bed, a mathematical model is established to describe the breach development and drainage process of tailings dam. The mathematical model is employed to simulate the breach process of a tailings dam. The calculated results show that the volume concentration of tailing sand changes greatly during the drainage process. At the early stage of drainage, the main part of the discharge materials is water, and the distribution of sand is mainly suspended load. At the later stage, the main part of the discharge materials is sand, and the distribution of sand is mainly bed load. The drainage process stops while the bed shear stress increment of discharge materials is less than the critical shear stress of sand.