Abstract: Abstract: In this study, a low specific speed mixed-flow pump with guide vanes was investigated experimentally. In order to study the internal flow characteristics in this mixed flow pump under different flow conditions, the internal flow of mixed flow pump under 0.8, 1.0, 1.2 times of the designed flow conditions were studied in this paper based on the particle image velocimetry (PIV) technology. Firstly, the external characteristics of mixed flow pump were acquired after conducting the repeated experiment. Then, the velocity distribution of internal flow field in mixed flow pump at inlet axial cross section of impeller, section clearance between impeller and guide vane and section in guide vane were measured. Moreover, the influence of flow rate change on the internal flow in mixed-flow pump was analyzed. The research results showed that the experimental repeatability of external characteristic was preferable and the result of experiment was reliable. The PIV experiment results showed that the distribution of velocity vector, velocity counter and vortex structure in the mixed-flow pump were greatly affected by the changing of the flow rate conditions. The velocity distribution of impeller inlet was consistent under three flow condition and the incoming flow from impeller inlet was along the axis direction. The velocity of impeller inlet increases with the flow rate increasing and the maximum speed can reach to 7.49 m/s. The velocity gradient of high speed zone from hub to rim was higher and the center of the distribution of contoured velocity was on the upper left corner which was decreasing to around continually. A high velocity area appeared vertically near the impeller under different flow rate conditions during the PIV experiment and the velocity of the fluids increased from the hub to the rim which was caused by the structure of the impeller blade. When the rotating speed was constant, the circumferential velocity of the blade near the rim was much bigger which had a great impact on the fluids nearby. The velocity distribution of gap flow field between impeller and guide vane was disordered which was affected by the rotor-stator interaction when the mixed pump operated. The anticlockwise vortex was formed at the inlet edge of guide vane near the hub and the fluid flowing from the impeller outlet was induced to the blade edge. With the increase in flow rate, the anticlockwise vortex tended to be smaller and the internal flow tended to be steady. Under the large flow rate condition, the maximum velocity of fluids almost reached 4.21 m/s in the middle of the axial clearance between the impeller and guide vanes. The flow field near the guide vane inlet was further influenced by the rotor-stator interaction and then the vortex structure was formed obviously which caused the flow passage congestion. Also, the fluids were structured by the end wall and the flow passages so as to the high velocity zones appeared near the end wall, and the velocity of fluids varied from 3.29 m/s to 3.95 m/s. At the same time, the velocity distribution of the fluids in the circumferential direction that flowed from the impeller also showed the movement axially and obliquely.The large scale vortex structure was formed at the guide vane inlet because of the effect of circular volute chamber. With the increase in flow rate, the high speed zone on edge of the guide vane moved to the downstream, the vortex structures at the guide vane inlet and outlet gradually disappeared while the flow losses decreased. The research results provided reference for revealing the internal flow characteristics of mixed flow pump. Moreover, the analysis of the internal flow fields could optimize the design of impeller of mixed flow pump.