Abstract: Construction of dams blocks the migration passage of migratory fishes, and the fishes passing by or stumbling into the turbine could be influenced by the turbine structure and its complicated hydraulic characteristics, possibly getting injured or even killed. Aiming at effective protection of fish resources, this paper presents a study on optimizing the number of turbine blades and the size of runner cone to lower the probability of juvenile fish injuries, using 3D full-passage numerical simulations of a large Kaplan turbine. Pressure and pressure gradient of the flows are used as threshold values for evaluating the injuries. The results show that an increase in the blade number or a decrease in the relative cone height leads to a higher probability of pressure and pressure gradient injuries on the passing-by juvenile fishes. This would helps design environmental friendly turbines to lower fish injury probability.