Abstract: As an important component of the side inlet/outlet, the diffusion section plays a crucial role in the bidirectional flow of pumped storage power stations. However, the influence of the vertical expansion mode of the diffusion section on the hydraulic characteristics of the inlet/outlet is still unclear at present. This paper studies the influence of different vertical expansion modes of the diffusion section on the hydraulic characteristics under bidirectional flow conditions at the inlet and outlet through numerical simulation methods. The aim is to clarify the degree of influence of the vertical expansion mode of the diffusion section on various hydraulic characteristics inside the inlet and outlet, provide the basis for selecting the vertical expansion mode of the diffusion section under different incoming flow conditions, and fill the gap in relevant design specifications. The results show that Under the outflow conditions, when the vertical expansion mode of the diffusion section is bidirectional symmetrical expansion of the top and bottom plates, the flow state of the diffusion section is better, the flow velocity distribution at the cross-section of the trash rack is the most uniform, and the flow velocity non-uniformity coefficients of the middle side holes are 1.870 and 1.655 respectively. The overall turbulent kinetic energy of the middle hole inside the diffusion section is the smallest, and the turbulent intensity at the cross-section of the trash rack is the smallest. The maximum turbulent intensities of the middle side holes are 0.41 and 0.29 respectively. Under the inflow condition, the vertical expansion mode of the diffusion section has a relatively small impact on the hydraulic characteristics of the inlet/outlet, and the overall turbulent kinetic energy inside the inlet/outlet is reduced by 7 times compared with the outflow condition. When the tailwater tunnel flows straight, it is recommended that the vertical expansion mode of the diffusion section be bidirectional symmetrical expansion of the top and bottom plates. When the tailwater tunnel flows inclined, it is recommended that the vertical expansion mode of the diffusion section be unidirectional expansion of the top plate, which is conducive to reducing the overall turbulent energy inside the inlet/outlet. The research results can fill the gap in the design specification regarding the selection of the vertical expansion mode of the diffusion section under different incoming flow conditions, and also provide a basis for the design of the diffusion section of the side inlet/outlet.