Abstract: For the study on the transient hydraulic characteristics and internal flow mechanism of the nuclear reactor coolant pump in the transient process from design operation conditions to off-design conditions, the variable flow transient characteristics of centrifugal pump impeller passageway were simulated by using CFX software. The results show that during the variable flow transition, the distribution of pressure pulsation of the nuclear reactor coolant pump along the circumference direction is non-uniform. The pressure pulsation trends to rise gradually to reach the maximum value and then fall, basically following a sine-shape changing law. The times of transient pressure fluctuation change are equal to the times of rotor-stator interference between the vane and the guide vane. The closer monitoring point to the intersection surface between the vane and the guide blade is, the greater the pressure fluctuation is. Because of the attack angle, the speed of the impeller passageway first falls and then rises. The guide vane not only transfers the kinetic energy to pressure energy, but also effectively reduces the pressure pulsation amplitude. During the transition to small flow, flow reducing causes the secondary backflow to occur near the outlet of impeller and in turn leads the amplitude of flow velocity variation in the flow channel of impeller to increase with flow decrease.