With software FLUENT, a k- ε turbulent model is used to simulate close Reynods equations and volume of fluid(VOF) method is adopted to track the free water surface, the flow state of overflow weir of Jiulongxia Hydropower Station is simulated by the method of three-dimensional numerical simulation according to the steady flow. The results show that the calculated results that the overflow weir discharge capacity, the water surface profile, the pressure distribution and velocity distribution are in good agreement with the measured data and numerical simulation can be used to some of the hydraulic structure calculations and designs.