Abstract: In order to study transient flow characteristics during the process from charging operation to small flow rate operation of a centrifugal charging pump in the nuclear power plant, CFX software was used for the simulation of transient process on the basis of RANS equation and RNG k-ε turbulence model. After test verification of the numerical method, the transient pressure and velocity characteristics of the charging pump were obtained from the simulation. The results show that the pressure in impellers, guide vanes and volute increase during the transient process. The vortexes generate and become serious with the flow rate decrease, so the flow pattern is getting worse and worse. As a result, the fluctuation amplitudes of pressure and velocity increase in this period. Because of the interference, fluctuations of pressure and velocity near the interface between impellers and guide vanes are stronger than that in other areas, and the fluctuations in inner channel is more serious than that of the outside channel in the volute.