The flow section area was controlled by increasing thickness of centrifugal impeller blade, so as to realize the non-overload characteristics of low specific speed sewage pump. It was shown from the external characteristic experimental results of prototype when the flow section area changed gently, the extreme point of shaft power curve appeared, and the units showed an obvious non-overload performance after the flow rate attained to 1.5 times of the designed flow. The experiments and prediction were also conducted to the section which perpendicular to the axis and passing by neutral position of impeller outlet. The experimental results showed that under the designed working conditions, the external characteristic experimental results were not only consistent with the computational results, but the PIV experimental data also had very good agreement with the internal flow field computation. At the same time, in the impeller channel, the relative velocity of pressure side was less than suction side due to the Coriolis effect, the flow field distributed uniformity with no obvious flow separation and vortex. The flow of each impeller channel was not in absolutely axial symmetrical distribution because of the rotor stator interaction between the impeller and volute.