The surface spillway is generally short for high arch dams due tothe thin dam crest. Discharge capacity can be affected if flaring gate piers are applied to surface spillway in high arch dams. Numerical simulation was carried out on the basis of the Dongzhuang water conservancy project to explore the flaring gate piers that can meet the design requirements for discharge capacity. The effects of plunge angles, contraction ratios, deflection angles on discharge capacity and the relationship between discharge capacity and Froude number along the surface spillway were studied. The results show that the discharge capacity of surface spillway decreases with the decrease of plunge angles, contraction ratios and the horizontal distance between starting flare and weir crest but it increases with the decrease of deflection angles. A comprehensive influence coefficient k_综 is proposed to reflect the effect of the shape parameters of flaring gate piers and water head over the weir. For k_综≥2, flaring gate piers have no effect on the discharge capacity of a surface spillway. For k_综<2, the effect on discharge capacity is gradually enhanced with a decreasing k_综. There is a knee point on the curve of Froude number along the surface spillway in the upstream of the starting flare. The discharge capacity is less affected by higher Froude number at the knee point.