In view of the problem that the trench wall for the diaphragm wall is prone to failure in the process of trench-digging in water rich sand pebble formation， two simulation test schemes are designed with either change of the flow velocity or the particle size ratio by analyzing the relevant research data and on the basis of defining the failure form and mechanism. The numerical simulation results show that the flow velocity has a great influence on the displacement of particles. The faster the flow velocity， the greater the particle displacement， the higher the porosity， and the greater the stress on the wall. The particle size ratio also has a great influence on the displacement of particles， and its effect is similar to that of flow velocity. When there is a lateral limit， the x-direction displacement of the particles will produce a z-direction component after reaching the limit， making the particles move upward. In practical works， it means that when the particle displacement reaches a certain degree， it will inevitably break through the mud film， resulting in the loss of fine particles in the soil， the increase of soil porosity， and finally the collapse of the trench wall.