To investigate the optimal range of the planar diffusion angle, this study employed a numerical simulation method to analyze the hydraulic characteristics of a lateral inlet/outlet in a pumped storage power station under different planar diffusion angles. The influence of the planar diffusion angle on the velocity distribution at the trash rack section, orifice flow distribution, and head loss of the inlet/outlet was analyzed. The results demonstrated that when the planar diffusion angle increased from 25° to 45°, the non-uniformity coefficient of the velocity at the trash rack section decreased gradually under the outflow condition, while the non-uniformity of the orifice flow distribution and head loss of the inlet/outlet increased gradually. The velocity distribution at the trash rack section, orifice flow distribution, and head loss of the inlet/outlet remained unchanged under the inflow condition. Considering the hydraulic indices of the inlet/outlet comprehensively, it is concluded that the optimal range for the planar diffusion angle is 30° to 40° when the average velocity at the trash rack section meets the design requirement. This range provides higher precision than the current specification and aligns with the diffusion angle range associated with minimal resistance coefficients in rectangular expanding conduits.