Abstract: For the study of the transient hydrodynamics of nuclear reactor coolant pump during stopping period, Pro/E software for 3-D modeling, CFX software based on Reynolds N-S equation and RNG k-ε equation were used to simulate the transient characteristics of nuclear reactor coolant pump impeller flow path during stopping period. The vortex in the impeller outlet is less than that in the impeller inlet. The vortex of the impeller outlet influenced by the static interference between the impeller and guide vane shows the periodic substantial fluctuations. The vortex in the junction between the pump body and outlet pipe is large, and the vortex in the opposite direction of the guide vane is the largest. The radial force of linear turning down model shows a periodic fluctuation and gradual decrease. Turning down model with the idler wheel has the same tendency as the linear turning down model though the change amplitude is less than that of radial force for linear turning down model. Especially while the parameter t/T is 0.6-1, the radial force is close to 0. The radial force of regular turning down model shows an irregular change. When the parameter t/T is 0.25, its radial force is the largest, which has great influence on the reliability of nuclear reactor coolant pump.