Abstract: The axial spacing between the inducer and impeller of a centrifugal pump influences the flow in the impeller section and the pump performance. To study these influences, we have numerically simulated the unsteady flows in a single-stage centrifugal pump with its pre-inducer and impeller installed at three different axial spacings, and analyzed the external characteristics, radial force on impeller, pressure fluctuations, and cavitation characteristics. Results show that in the range of the axial spacings examined, an increase in spacing leads to an increase in working head and efficiency, with the maximum variations being 1.8% and 4.3% respectively. Variation in the axial spacing modifies the interaction between the impeller and guide vanes, and thus it can influence the magnitude and direction of the radial force on the impeller to some extent. For cavitation performance, an increase in the spacing from 0.05D1 to 0.1D1 results in a significant improvement, while a further increase up to 0.15D1 shows a negative effect. In addition, the frequency and amplitude in pressure fluctuations at the impeller inlet is affected directly.