The weir surface curve of the spillway surface opening in high gravity dams is one of the important factors affecting the hydraulic characteristics of the discharged water and the stability of the spillway structure. Due to the steep weir surface curve, local negative pressure accumulates on the weir surface during the prototype flood discharge of the spillway surface opening. To avoid cavitation and erosion, a three-dimensional numerical model of the spillway surface opening of a high gravity dam was established for the optimization of weir surface curve. The RNG k-ε model was used to simulate turbulence, and the volume of fluid(VOF) method was adopted to track the free surface. The flood discharge of the spillway surface opening in a high gravity dam was simulated and the results were compared with that of the hydraulic model test, which demonstrated the accuracy of the simulation. Three optimization schemes of the WES curve equation were further proposed to study the influence of different schemes on the negative pressure of the spillway surface opening. The research results show that the length and value of the negative pressure on the weir surface bottom plate both decrease, as the weir surface curve becomes gentler. Scheme 1 has the smallest negative pressure, although the discharge capacity slightly decreases, it still meets the design requirements. Moreover, as the weir surface becomes gentler, the flow lines in the curve section become smoother, the flow velocity at the same section decreases, so does the bottom flow velocity gradient. At the same water depth, the entrained air concentration increases. Therefore, scheme 1 is selected as the final optimized shape.