To investigate the effects of the diffuser geometry and its connection method with the bottom of the shaft-type inlet/outlet on the outflow characteristics, a three-dimensional numerical model of the flow field was established for the upper reservoir shaft-type inlet/outlet of a pumped storage power station in China. The Realizable k-ε turbulence model was employed to calculate and analyze the influence of the diffuser geometry and its connection with the bottom of the shaft-type inlet/outlet on outflow uniformity and head loss. The results indicate that a conical diffuser with an appropriate diffusion angle is more conducive to achieving uniform flow distribution among the orifices of the shaft-type inlet/outlet and reducing head loss compared to an elliptical diffuser. Although adding an arc connection at the junction between the conical diffuser and the bottom of the orifices can reduce head loss, it increases the non-uniformity of flow distribution among the orifices.