A new herringbone water-sediment separation structure was designed to separate coarse sediment from debris flow. The width of water-sediment separation grids is key to separation effectiveness. A formula for grids width was proposed by theoretical analysis,A optimal width discriminant of grids was gived by theoretical analysis,that is,the movement time of sediment larger than design separation diameter sliding down grids is equals to that of the residual debris getting through grids. And then,model tests were developed to revise the formula. Only gravitational effect is considered,based on the above analysis,the theoretical formula consisting of the width and depth of overflow gate and the incline angle of grids is established. Tests results demonstrate that measured values are 1. 53 times larger than theoretical ones,and the correlation coefficient is 0. 93. The coefficient can be used to revise theoretical models.