In order to investigate the instabilities of cavitation at low flow rate of centrifugal pump, with the development of cavitation in a centrifugal pump, the characteristics of the internal flow, inlet and outlet pressure pulsations of the pump were researched through numerical simulation and experiment. It was found that the rotorstator interaction between the volute tongue and impeller made a significant influence on the internal flow instabilities of a centrifugal pump under low flow rate condition. The vortexes generated by the influence of cavitation and volute tongue presented in opposite directions. The dominant frequency of the pump inlet pressure pulsations was two times shaft frequency, which migrated to low frequency with certain fluctuations with the development of cavitation. The peak which existed at two times blade passing frequency of the pump inlet pressure pulsations vanished when the cavitation developed to a certain degree. The dominant frequency of the pump outlet pressure pulsations was the blade passing frequency resulted from the rotorstator interaction between the volute tongue and impeller. The peak which existed at two times shaft frequency presented intensive fluctuations. The broadband pulsations of both pump inlet and outlet pressure pulsations changed obviously with the decrease of available net positive suction head.