Abstract: For flow distribution in the side intake/outlet tunnel of pumped storage plants, the design guide stipulates that the difference of flow between the middle channel and its adjacent side channel of the diffusion section should be less than 10%, but actual designs are unable to meet this rule. In this work, flows in the side intake/outlet tunnel of a pumped storage plant have been numerically simulated using a turbulent flow model, focusing on the variations in flow distribution over different intake/outlet tunnels by adjusting three parameters: the width ratio of these two channels in the diffusion section with guide piers, the location of the middle guide pier’s head, and the length of this section. Results show that the flow non-uniformity can be improved effectively by optimizing the three design parameters but it is yet difficult to meet the less-than-10% rule. The length of diffusion section has a relatively less effect on the flow uniformity. The results would help the design of side intake/outlet of similar power plants.