The internal flow of a volute-type discharge passage in a mixed-low pumping system was numerically simulated with the RNG k-εturbulence model of the time-average closed Navier-Stokes equations and on the basis of analyzing the flow patterns in key cross sections, the hydraulic characteristics of the discharge passage was predicted. The computed results showed that the internal flow field is very complicated with flow separation and vortex in some regions, and the velocity distribution uniformity are quite poor. The uniformity curves of axial velocities discharge in the outlet cross-sections of the volute and the passage are protruding ones while the bias angle curves for the same cross-sections are concave ones, they would reach their maximums and minimums respectively at the discharge corresponding to the best efficiency point of the pumping system. When the pumping system operated under off-design conditions on the larger flow rate side, the head loss of the passage would increase rapidly to cause the efficiency of pumping system drop sharply.