Taking the side inlet/outlet of an upper reservoir without adjustment section as the research object, the Reynolds stress model was used to explore the influence of diffusion section length on the hydraulic characteristics of the inlet/outlet, and the recommended value of diffusion section length was given from the perspective of hydraulics. The results show that the length of the diffusion section has a significant influence on the water flow movement of the side inlet/outlet, and the diffusion section should not be too long or too short. The flow velocity distribution along the inlet/outlet can only be affected by the length of the diffusion section under the pumping condition, and there is basically no influence under the power generation condition. For other hydraulic indicators such as head loss and flow distribution at each orifice, the length of the diffusion section will have a greater impact on it when the flow is in both directions, and with the increase of the length of the diffusion section, the hydraulic indicators will first become better and then worse. For the body type of side inlet/outlet without adjustment section, when the length of the diffusion section is 5.7 to 6.0 times the tunnel diameter, the hydraulic indicators meet the requirements of the specification. When the length of the diffusion section is 5.8 times the tunnel diameter, the hydraulic characteristics of the inlet/outlet are greatly improved, and the water flow pattern is better.