Abstract: To explore the hydraulic performance of a side inlet/outlet with vertical bidirectional expansion in the diffusion section, numerical simulations were conducted to analyze the velocity distribution at the trash rack section and the internal flow patterns under a constant total vertical expansion angle. The advantages of vertical bidirectional expansion in the diffusion section were also examined. The results show that: ① Under outflow conditions, with the decrease of the roof expansion angle and the increase of the floor expansion angle, the mainstream at the trash rack section shifts from the bottom toward the top, and the velocity nonuniformity coefficient decreases first and then increases. ② Under inflow conditions, the velocity distribution at the trash rack section remains essentially consistent across different vertical bidirectional expansion conditions. ③ When the total vertical expansion angle is fixed, the ratio of the roof expansion angle to the floor expansion angle should be in the range of 0.5—2.0. ④ The vertical bidirectional expansion in the diffusion section effectively improves the velocity distribution at the trash rack section, reduces inlet/outlet head loss and decreases the excavation volume. These findings provide new insights for the design of inlet/outlet structures.