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After structural modification of a submersible sewage pump with a specific speed (ns ) of 60, an experimental study combined with numerical simulation and PIV measurements was carried out in order to analyze the internal flow of the volute. The calculated region was spitted by structure grid and the numerical simulation of whole flow field was performed. The results showed that due to the mouth ring leakage, turbulent kinetic energy was the greatest near this region, and the turbulent kinetic energy in the volute was greater than one in the impeller at the design condition. In addition, comparisons between experiment results of pump performance, PIV test results and CFD computations showed that they were in good agreement, which also validated the numerical result. At last the following conclusions were obtained: affected by the fact that the angle between relative velocity and circumferential direction was became larger with the increasing of the flow rate, when the flow rate was 0.6 Qopt and Qopt , partial fluid in the eighth section return to the first section through the volute, so, there were great velocity and velocity gradient in the region between the tongue and volute inlet. For 1.4 Qopt , this phenomenon disappeared, and at the region near the eighth section, its velocity was maximum, which leaded to a larger velocity gradient. Besides, at the flow rate of 1.4 Qopt , in the region between the first section and the fifth section, three high-speed wake regions from the impeller outlet could be obviously observed.