Abstract: Abstract: Transient performance of pumps during transient operating periods, such as startup and the regulating valve, has drawn more and more attention recently due to growing engineering needs. It is impossible for a pump to work at a working point forever, namely that the switching process among different working points must be existent. In order to reveal the transient characteristics of a prototype centrifugal pump in the transient process of the decreasing flow rate by the regulating discharge valve, a low specific-speed centrifugal pump was chosen as the research object to investigate by using the theoretical analysis and numerical simulation, respectively. Through the research, the external performance and internal flow field of the pump model are obtained during the transient operating period. Based on the deduced generalized Euler equation of turbomachinery, the additional transient theory head of the pump model are quantitatively calculated and analyzed in the process of decreasing the flow rate. Results show that under the same conditions, the flow rate after the regulating discharge valve is smaller, the greater the additional theory head, which manifests the transient effect that is more obvious. Meanwhile, the transient effect at the later stage is more remarkable than that of former stage. Subsequently, the RNG k-? turbulence model, sliding mesh, and user defined functions (UDF) are employed to simulate the three-dimensional unsteady viscous incompressible flow in the centrifugal pump during the rapid regulating flow rate. The results show that the rotor-stator interaction plays a dominant role in the fluctuating characteristics of flow parameters at the pump outlet, while the influence on flow parameters at the pump inlet is not very obvious. Moreover, compared with the influence on the condition of the large flow rate, the rotor-stator interaction has a more remarkable effect on the condition of the small flow rate. The predicted pump head is smallest when the relative position between blade and tongue is nearest. Similarly, the predicted head is largest when the tongue is at just after the middle of impeller channel. In a rotational cycle, choosing two relative positions, namely 0.225 T and 0.825 T as the single steady initial phase, would obtain the best numerical prediction accuracy. The different flow components and viscous effect together make the characteristics of the axial velocity distribution in the impeller and the volute opposite. The flow acceleration effect is the most important reason that the flow field evolution in transient calculation lags behind that of the quasisteady calculation as a whole.