Abstract: To study the flow characteristics in the spiral casing of centrifugal pump, steady and unsteady flow fields were measured by PIV under the modes of internal synchronization and external synchronization, respectively. Flow phenomenon and regular pattern in the spiral casing were analyzed in detail. The results show that the velocity in the spiral casing distributes homogeneously in axial direction, and decreases with the increase of angle of circumference. The velocity dividing line between the impeller and the spiral casing is clear. 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. It is obvious that the unsteady flow in the spiral casing is periodic. On the VIII section, 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 results provide valuable information for the design of spiral casing and the experimental study on flow in the centrifugal pump.