Centrifugal pump is a kind of important industrial installation for fluid delivery. The research on the unsteady flow in centrifugal pump is very meaningful to reducing vibration. Particle image velocimetry (PIV) system and test pump designed for PIV measurement were introduced. The experimental scheme and the methods of numerical simulation were discussed. PIV technique was used to measure the unsteady velocity field near the volute tongue under the mode of external synchronization. The unsteady pressure field was simulated by using Sliding Mesh (SM) model provided by Fluent. The results show that the velocity and pressure fluctuate periodically with the rotation of impeller. Partial fluid flows back to the impeller passage and the velocity in the inlet of diffusion tube decreases significantly due to shunt effect of the volute tongue. On the section VIII, the magnitude and fluctuation range of velocity show a decreasing trend in radial direction. The fluctuation of circumferential velocity is related to the position of high-speed flow in impeller passage, and the fluctuation of radial velocity is influenced by blade interference and Coriolis force. The static pressure increases and the dynamic pressure decreases in the radial direction of volute. The velocity and the pressure on the section VIII and the outlet total pressure fluctuate intensively when the blade tail end passes the section VIII and the volute tongue. The vibration of pump can be reduced by increasing the volute tongue mounting angle and decreasing the blade outlet mounting angle properly.