The simulation of cavitating flows in hydraulic runner for the hydraulic optimal design was presented by using the two-phase flow method through the solution of the time-average Navier-Stokes equations based on the mass transport equation. The results show that the efficiency apparently increases from 93.4% to 94.7% when the wrap angle of the blade enlarges and the trailing edges of the blade extend. The pressure on the blades is well distributed and the critical cavitation coefficient at optimum operating condition is lower than unmodified, 0.03, and the cavity domain are also decreased. Finally, the cavitation performance of runner is improved. The computational results, which agree with experimental data, provide a correct direction for further optimal design for the runner.