To study the effect of gill-piece spacing on velocity field and sediment distribution in gill-piece separation device, 3D numerical simulation based on laminar model, Eulerian model and Phase Coupled SIMPLE algorithm provided by Fluent software was conducted in simulating water-sediment flow field of different gill-piece spacing under hydrostatic condition. The velocity field and the distribution of sediment concentration were compared and analyzed according to the numerical results. It indicated that the velocity distribution was different under different gill-piece spacing. The effect on velocity flow field from sediment flow in the sediment channel and clear water flow in water channel would get greater as the gill-piece spacing became greater. When the gill-piece spacing was smaller, the precipitation effect would be better in gill-piece separation device. Taking the average velocity of sediment and clear water as the examination index and considering the characteristics of flow field, effect of water-sediment separation, and cost of making gill-piece separation device, the best gill-piece spacing was proposed as d=5 cm.