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This study numerically investigated the flow return system performances of two-stage high pressure fans. The parameters include inflow angle, length, exit width of vaneless diffuser and the blade number of deswirl vane. This study used the tested impeller to verify the accuracy of numerical calculations, with good consistency. The analytic results showed that the overall pressure loss of impeller and flow return system is effectively reduced when the inflow angle of vaneless diffuser is 15 ı and tangential to wheel velocity ratio is 0.46. A vaneless diffuser was required and its length referred to the maximum outer diameter of the volute casing. The performance of a contracted-wall vaneless diffuser was not improved over that of a parallel-wall one. The suggested number of blades in a deswirl vane was 12. The downstream affects the upstream in several areas inside the flow return system, thus, the flow return system must be calculated integrally.