A new tailings pond of Pingchuan iron Co., Ltd. located in Yanyuan county of Sichuan province has been used for the project instance in this research. The dry slope’s geometric features and particles’ distribution rule of the tailings dam are obtained by a model test of building tailings dam, and so do the mechanical properties of tailings by the soil test. Based on the discrete element theory, we obtain the mesomechanics parameters by comparing the results of biax test numerical simulation with PFC 2D to the mechanical properties obtained by the soil test. Also the contact forces among particles, particles’ motion and deformation characteristics of tailings dam model have been studied. The results clearly show that: 1)There are some big and stable force chains in initial dam, and which directions are almost inclined upward with 45°. The particles’ contact forces in subdam grow with the increase of depth, and the contact forces near the embossment point of bedrock are much bigger and more intensive than those in other area. 2)The particles’ motions in initial dam are insignificant. The motions of particles in subdam near the inner slope of initial dam are significant, which directions are almost upward along the inner slope of initial dam. Some particles in the surface of subdam tumble along the slope, which motions are much significant. There is a typical slip plane in subdam, and the motions of the particles above the slip plane are much obvious. 3)The structural change of initial dam is small, but the structural deformation of subdam near initial dam’s inner slope is much significant.