To optimize the adjustment-section length for the side inlet/outlet of a pumped storage power station, this paper takes the side inlet/outlet of the lower reservoir of the Yabuli Pumped Storage Power Station as its research object, and uses the numerical simulation method to discuss the influence of the ratio of the adjustment-section length to the diffusion-section length on the hydraulic characteristics of the side inlet/outlet, while keeping the total length of the inlet/outlet fixed. Under the outflow condition, the results show that as the ratio of adjustment-section to diffusion-section length rises, the mainstream position at the trash rack section of the middle channel gradually shifts from the upper-middle part of the orifice to the bottom of the orifice, while that of the side channel does not change significantly. The velocity non-uniformity coefficients at the trash rack section of both the middle and side channels first decrease and then increase. When the adjustment-section length is 50% of the diffusion-section length, the velocity non-uniformity coefficient at the trash rack section reaches its minimum, and there is no negative velocity at the trash rack section. Under the inflow condition, the hydraulic indices of the inlet/outlet are essentially unaffected by variations in adjustment-section length. Therefore, with the total length of the inlet/outlet fixed, all hydraulic indices of the inlet/outlet are relatively better when the adjustment-section length is 50% of the diffusion-section length.