Abstract: To improve the flow characteristics in the runner-headcover gap, this paper proposes a throttling device at the pressure equalizing pipe for the pump-turbine of Meizhou Pumped Storage Power Station, and investigates its effects on flow regime and air admission in the gap.The study adopts the computational fluid dynamics (CFD) method, selecting the SST k-turbulence model and the Volume of Fluid (VOF) multiphase flow model to conduct numerical simulations of the runner-head cover gap with throttling orifices of different diameters (relative diameters d/D = 0.238, 0.298, 0.357, 0.476, and the case without a throttling orifice). The analysis results show that a smaller throttling orifice size induces vortices and backflow in the flow pattern at the pressure equalizing pipe. The throttling orifice size has a significant influence on the gap leakage flow rate and the axial force on the lower wall; when d/D exceeds 0.357, the leakage flow rate can be effectively reduced with little impact on the axial force on the lower wall. In addition, the throttling orifice size exerts a considerable effect on the air admission performance. The overall gas volume fraction and the gas volume fraction in the outer cavity of the gap reach the maximum at d/D = 0.476, indicating the best air admission effect at this orifice size. A comprehensive consideration of the gap flow and air admission performance reveals that d/D = 0.476 is the optimal size. The research results of this paper provide a theoretical basis for optimizing the structural design of the pressure equalizing pipe and enhancing the safe and stable operation of the unit, which is of important engineering value.