Abstract: In practical engineering of pumped storage power station, water tunnels often feature horizontal bends influenced by terrain, leading to significant flow differences between the channels at the inlet/outlet. This study focuses on a lateral inlet/outlet structure with a horizontal bend angle of 29.7°. Three-dimensional numerical simulations were used to analyze the evolution of Dean vortex structures within the bend flow. Based on the characteristics of the Dean vortex, two solutions to address the flow imbalance in the lateral inlet/outlet were proposed and discussed. The results show that the Dean vortex structure causes a "crescent-shaped" distribution of the core flow within the tunnel cross-section, with a low-velocity zone on the inner side of the bend, causing an uneven velocity distribution. Reducing the tunnel bend radius and adding guide vanes can lower the flow velocity on the bias-flow side of the tunnel and alleviate the uneven velocity distribution. However, this also increases the flow rate in the middle channel on the bias-flow side. On this basis, moving the central pier forward can balance the flow rates in each channel and achieve uniform flow distribution at the inlet and outlet. Nevertheless, these combined measures result in additional head loss. Alternatively, adjusting the tunnel axis to induce Dean vortices at the top and bottom of the tunnel counteracts the original central Dean vortex, significantly improving the uneven velocity distribution and achieving balanced flow at the inlet/outlet. The research results provide valuable guidance for the design of lateral inlet/outlet structures with horizontal bends.