Abstract: Cavitation bubbles are easy to block the long, narrow inlet passage of a low specific speed centrifugal pump and always induce severe noise and vibration, thus imposing significant influence on its operational safety and stability. In this study, unsteady cavitating flows in a centrifugal pump at a specific speed of 45 are simulated using a RNG k-ε turbulence model and a Zwart-Gerber-Belamri cavitation model. We have well predicted the NPSH-head curves of the pump in various working conditions with a good agreement with the experimental results and obtained the full flow fields and the characteristics of unsteady cavitation bubbles in the pump. Results show that these bubbles behavior differently in different pump working conditions. At a low flow rate (0.8Qd), cavitation bubbles in each impellor passage show an obviously periodic cycle, which is characterized by three stages of shrink, stabilization and expansion at NPSH = 4.75 m, while by four stages of stabilization, split, growth and shrink at NPSH = 3.61 m. At the rated flow (1.0Qd) cavitation bubbles in the impellor passages are relatively periodic and uniformly distributed, while at a larger flow rate (1.2Qd) they are nonuniform showing an alternative passage blocking phenomenon. As the flow rate is increasing, the radial force on the impeller is distributed in a regular five-pointed star shape until cavitation is started, and along with the development of cavitation it develops into disordered patterns owing to the asymmetrical distribution of cavitation spots over the impeller.