Abstract: Aiming at the instability problem in the ″S″ characteristic region of the pump-turbine, a modified movable guide vane airfoil was proposed to improve the performance. Taking a pump-turbine model machine as the research object, the SST k-ω turbulence model was used to carry out three-dimensional numerical calculations on the whole flow channel of the pump-turbine. By modifying the design of the movable guide vane airfoil, a Q-n curve was obtained, and the improvement of the ″S″ characteristics of the unit was analyzed. The calculation results were compared with the test results, and the pressure pulsation analysis was carried out in the bladeless area of the unit before and after the modification of the movable guide vane airfoil. The results show that under the premise of ensuring that the overall efficiency is still maintained near 92%, the new movable guide vane airfoil still has an improved effect on the ″S″ characteristics of the unit. The pressure pulsation in the bladeless zone is mainly caused by the aggregation of the flow in the active guide vane area and the disturbance of the rotator and stator interaction with the runner blades. The modified movable guide vanes reduce the amplitude of pressure pulsation in bladeless zone and improve the grid connection stability of the pump-turbine during operation.