Using particle flow code(PFC) based on discrete element theory and combining with the small-scale model tests，the piping in sandy soils is simulated with fluid-solid coupling. The porosity of sample，fluid velocity，contact number of particles，pathway of movable particles and loss fraction of eroded particles and so on are traced and recorded in the analytical model. The results show that the movable particles are eroded gradually；and the fluid velocity and porosity of sample increase gradually with the increase of water head；the permeability of sample changes，which will be influenced greatly by the response of fluid velocity and water head. The results reveal that the geometric and hydraulic features of system change dynamically and nonlinearly，which disclose their interactive behaviors during piping. The results also show that the loss fraction of eroded particles decreases along the seepage flow path；the small particles remove randomly；and the leak passages of piping are formed randomly when piping，which are the outcome of the complicated interactions between water and soil. The violent fluctuation of contact number of movable particles shows the violent motion of small particles during piping；and the stress field of soils is not influenced by particle erosion fundamentally. The availability and rationality of the proposed numerical method is verified by comparing the numerical solutions with the results of relevant physical model tests. The results are valuable to further probing into the piping mechanism in sandy soils.