Abstract: Based on the Realizable k-ε turbulence model,taking the pump-turbine of a domestic pumped storage power station as the research object,this paper improves the circular radius r of the guide vane head and analyzes the runaway characteristics of the pumpturbine with the improved circular radius of the guide vane head,giving full play to the core role of pump-turbine in pumped storage.The results show that reducing the head circular radius r to（r-0.313）mm has a better effect on the“S”characteristics of the pumpturbine. The high-speed water ring in the vaneless area is weakened obviously under the runaway condition. When the fluid enters the runner blade channel from the guide vane,the velocity of the fluid increases gradually near the outlet of the guide vane and reaches the maximum in the vaneless area due to the influence of the high-speed water ring. Then the velocity decreases gradually and enters the runner and keeps flowing at a high speed. In the runaway condition,the vortex at the inlet of the runner blade of the 1-5 runners is gradually flattened along the runner and spreads back to the runner blades. The pressure change near the vortex at the inlet of the runner blade has a overall downward trend and a peak appears at the inlet of the runner blade. The pressure near the vortex at the inlet of the runner blade varies with the displacement and strength of the vortex. A large number of vortexes are distributed in the straight conical section of the draft tube under the runaway condition,and the vortexes move gradually to the diffusion section along the elbow section.The backflow is mainly concentrated near the side wall of the diffusion section of the draft tube. When 300-500 mm away from the central axis of the runner,the backflow gradually decreases.