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As relative motion is existing between the impeller and volute, periodical pressure fluctuations is produced by the flow interaction of the impeller and volute. Numerical simulations showed that obvious high speed regions were formed along the direction of the absolute velocity in the casing at the down-stream of the trailing edges. The pressure fluctuations in casing present periodical features, have five peaks and five valleys according to the number of vanes. Frequency spectrum showed that the dominate frequency of the pressure fluctuation was the blade passing frequency. Pressure acquisition in experimental test was executed by a high-frequency pressure sensor installed in the pump outlet flange. FFT technology was used to analyze the measured signals, and it also showed that the dominate frequency was the blade passing frequency. A new technology to measure the pump rotating speed was developed according to the feature of the rotorstator interaction. The original contents were those pressure signals at the pump outlet. The dominate frequency of the contents was calculated by the technology of FFT, and then the rotating speed was calculated.