Abstract: The side inlet/outlet of a pumped storage power station is featured with bi-directional flows. If the passage is not properly shaped in design, flow separation will occur in its diffuser section that causes non-uniform velocity distribution and even back flows in the section of trash racks in outflow operation. This paper demonstrates the importance of vertical expansion angle ? and horizontal expansion angle ? to flow separation through numerical simulations using a realizable k-? turbulence model. Results show that ? plays a determinative role in weakening flow separation and eliminating back flow. Based on that, we discuss the influence of the angle ? on flow separation and velocity distribution in the condition of fixing the area of orifices under the required velocity in the trash rack section. This study suggests that flow separation or back flow will not occur if ? is no larger than 2.0° and the Reynolds number is less than 2.7?107.