In order to optimize the hydraulic characteristics of a large-slope vertical slot fishway pool, a three-dimensional hydrodynamic numerical model was constructed on the basis of physical model validation. Numerical simulation was carried out to analyze the hydraulic distributions of the pool with or without the vertical slot sill and with different installation heights of the vertical slot sill, considering five types of large slope gradients. The results demonstrate that the pool’s maximum flow velocity and average turbulent kinetic energy are successfully reduced by setting the sill at the vertical slot. For a 1/20 slope, the maximum flow velocity of the pool is reduced by 10.16% to 14.84%, the average turbulent kinetic energy is dropped by 21.88% to 42.19%, and the mainstream flow velocity decays at a rate of 61.24% to 61.82%. A Ω-shape continuous flow with small flow velocity and low turbulence is formed in the pool.The installation heights of the vertical slot sill can affect the velocity distribution in the water depth above and below the sill, but has no significant effect on the distribution regimes of velocity, turbulence kinetic energy and strain rate in the water depth above and below the sill.