Abstract: The numerical simulation calculation of the transient flow characteristics of nuclear reactor coolant pump in the recessive cavitation transition process in the nuclear reactor coolant pump impeller passage is conducted by CFX,and the transient flow characteristics of nuclear reactor coolant pump in the transition process from reducing the inlet pressure at cavitation-born conditions to NPSHc condition is studied and analyzed.The flow field analysis shows that,in the recessive cavitation transition process,the speed diversification at the inlet is relative to the bubble increasing,and makes the speed near the blade entrance increase when the bubble phase region becomes larger.The bubble generation and collapse will affect the the speed fluctuation near the entrance.The vorticity close to the blade entrance gradually increasing is influenced by the bubble phase,and the collapse of bubble generated by cavitation will reduce the vorticity from the collapse to impeller outlet.Pump asymmetric structure causes the asymmetry of the flow,velocity and outlet pressure distribution within every impeller flow passage,which cause the asymmetry of the transient radial force.From the dimensionless t/T =0.6,the bubble phase starts to have impact on the impeller transient radial force,and results in the irregular fluctuations.