Abstract: Three-dimensional numerical simulations were conducted for the water-sediment two-phase flow in a gill-sheet separation device with a laminar model and an Euler particle multiphase flow model solved by the phase-coupled SIMPLE (PC-SIMPLE) algorithm to study the effect of sediment concentration of muddy water on velocity field and characteristics of sediment distribution. The results show that sediment concentration has no effect on velocity distribution of the flow in this device but a higher concentration leads to a greater average velocity of the flow and a higher average concentration. A lower concentration of muddy water improves the effect of water-sediment separation. In the height direction of the device,water-sediment separation is faster at the locations closer to the top of the device, and at these locations it is relatively easier to obtain clean water.