Abstract: In integrated small power reactors, several subsystems are connected in parallel with the flow rate provided by a centrifugal pump so that the system structures are more simple and compact. The pipeline resistance characteristics of each subsystem vary when switching over the running modes, which affects the pump working conditions and the flow distributions in each subsystem as a results. The influence of the subsystem pipeline resistance characteristic variation on the pump working parameters and the flow distributions in each subsystem were calculated and analyzed in certain running conditions. By pass control and pump frequency converting speed regulating methods were adopted to sustain the necessary flow and pump head in each subsystem. The pump frequency converting speed regulating method is relatively more reliable in reducing the mutual influence on the flow distribution of each subsystem when the pipeline resistance varies in certain branch.