Abstract: Abstract: In order to analyze the vibration characteristic of a multistage centrifugal pump while it is operating, the unsteady flow field at rated flow condition of a two-stage centrifugal pump was simulated with ANSYS CFX software based on the standard k-ε turbulence model to investigate the pressure fluctuation at two different points: at the outlet of the impeller and in the radial guide vane. The radial hydraulic force and torque on the impeller were obtained at the same time. Before simulating, a three-dimensional computational domain was established in UG8.0, and then the structural grid was generated through ICEM CFD software. The influence of grid number on pump character was analyzed as well to reduce the simulative error caused by grid number. Finally, the suitable grid number was determined by considering the simulative accuracy and the calculation of time. After the calculation of unsteady flow, the evident pressure pulsation was found both in the impeller outlet and the radial guide vane. The numerical results of static pressure at monitoring point were analyzed using a Fast Fourier Transform (FFT). The analysis indicates that the blade passing frequency dominated the periodic pressure fluctuation. The most evident pressure fluctuation occurred near the impeller outlet; the amplitude of pressure pulsation decreased with the flow in radial guide vane, and the regulation of pulsation was becoming distinct gradually. The amplitude of pressure fluctuation in the spiral section of the radial guide vane also decreased with increasing distance between the monitor point and the impeller outlet. Pressure and frequency fluctuations show the radial hydraulic force on the impeller. The maximum force on the impeller was about three times as much as the minimum force on impeller, and the magnitude of force on blade pressure surface would increased when one blade was passing a frontage vane. There were differences between the average magnitude of force on the blade pressure surface with that on the blade suction surface and that on the shroud surface. The distribution of the radial hydraulic force vector coordinates presents almost as a circle; the torque on the impeller pulsates with the frequency of blades passing frontage guide vanes. The rotor-stator interaction (RSI) between impeller and guide vane is the major factor which affects the fluctuation of pressure and torque. Further understanding for the relationship between the structure of a multistage pump (such as: blade numbers, rotor speed, clearance between rotor and stator, etc.) and regulation of pressure pulsation will contribute to improving vibration characteristic of pump. A performance characteristic test of the pump was also conducted in the opening experimental platform. The simulation results agree well with the test results, so results from numerical calculation are available.