In order to explore the internal flow characteristics of mixed-flow pump and optimize and improve the design method of the mixed-flow impeller and guide vane, the internal flow field of mixed-flow pump was measured based on the particle image velocimetry (PIV). The streamline and velocity distribution of impeller section at different phases and the vorticity distribution under small flow rate conditions were analyzed. The results showed that radial trend of flow was appeared in the impeller due to influence of vortex flow on the blade pressure surface and flow separation of suction surface under small flow rate conditions, and the flow was turbulent. With the increase of flow rate, the streamline was turned gradually in the axial direction and flows along with the contour line of hub. Under the large flow rate conditions, the vortex structures near blade pressure surface and close to the wall were obvious. At 0.6 times of the working condition, when impeller inlet came to the shooting section, clockwise rotation vortex was formed within the impeller; when impeller outlet came to the shooting section, the positive vorticity concentrated area appeared near the guide vane inlet edge and this region was moved in the inlet direction with rotation of impeller. When the blade outlet kept away from the shooting section, the negative vorticity concentrated area was appeared near the guide vane inlet. The causes of mixed-flow pump energy loss were revealed during the process of rotor-stator interaction between impeller and guide vane. The research results provided reference for revealing the internal flow characteristics and expanding the operation ranges of the mixed-flow pump.