Abstract: In order to study the influence of clocking effect on the centrifugal pump, the CFD method was applied to simulate the inner flow in the centrifugal pump at its design flow rate under different guide vane clocking positions. The centrifugal pump mainly consisted of volute, guide vane and impeller. The physical model was made of transparent organic glass. The clocking positions were changed by changing the circumferential connection between guide vane and volute. First, we defined the position at which the angle between guide vane blade trailing edge and volute tongue was 0° as Clocking0, and the clocking position number increases by 1 every increase of 10° clockwise. The calculation result at Clocking1 position was compared with its experimental value. The experiment was conducted in a closed test platform. The pump head and efficiency error between experimental and calculated results were less than 3%, which indicated the CFD calculation method predicted the performance of centrifugal pump relatively well such that it can be used for predicting the characteristics of other clocking positions. The external characteristics and internal flow features variations of the pump versus clocking positions were obtained. The pressure fluctuation at the tongue region and the radial hydraulic force on the impeller at different clocking positions were analyzed as well. The results indicated that with the increase of clocking position, the pump head and efficiency increased and then decreased, reaching the maximum value near Clocking2. The minimum value of pump head and efficiency was at Clocking0. The pump head at Clocking2 was 0.6m greater than that of Clocking0, and the efficiency was about 5% greater. The average relative pressure differences on each cross section of pump volute were analyzed, which indicated that the pressure variations were within 20% with the increase of clocking positions. The analysis on frequency fluctuations and frequency spectra of pressure fluctuations at locations around tongue for different clocking positions revealed that clocking effect mainly affected one and two times the blade passing frequency at the tongue region. When the clocking position number increased, both the dominant frequency and the amplitude of the pressure fluctuation increased and then decreased. The minimum value of the pressure fluctuation amplitude was at Clocking1, the maximum value of pressure fluctuation amplitude was at Clocking4, and the minimum value was 70% of the maximum. In addition, the structures of pressure fluctuation translate periodically with the sequence of clocking positions. The analysis of unsteady radial hydraulic force on the impeller showed that with increasing clocking position, the value of radial hydraulic force first decreased and then increased, the maximum value was at Clocking4, and the minimum was at Clocking1. The study demonstrates that the guide vane clocking position has the very important impact on centrifugal pump and it provides a reference for the guide vane design in centrifugal pumps.