Abstract: In order to study influence of the velocity circulation at the guide vane outlet on the hydraulic performance of outlet conduit for a large pump system quantitatively, the methods to calculate velocity circulation and to measure the average angular velocity for the flow at the guide vane outlet of the pump system were put forward，the influence of circulation on the hydraulic losses of both siphon and straight outlet conduit were studied numerically and experimentally. The results indicated that the hydraulic loss of the outlet conduit was obviously influenced by the circulation at the guide vane outlet. There was an optimum circulation for the hydraulic loss of the outlet conduit to be minimized, the optimum circulation for siphon and straight outlet conduit was 0.972 and 1.308 m2/s respectively. The hydraulic losses calculated of the siphon and straight conduit under the condition of the optimal circulation were respectively lower 0.126 and 0.180 m than those under the condition of zero circulation. This study above could be helpful to optimal hydraulic design both for outlet conduit and guide vane of an axial-flow pump.