Stall in centrifugal pumps is a complicated flow phenomenon, which is detrimental to the pumps' safety and stable operation. Using a high-frequency particle image velocimetry (PIV) system (f = 10k Hz) and a bench-scale refractive index matching experimental setup, two measurement methods are introduced to observe the dynamic stall inception and evolution. In the first method, the flow rate was continuously reduced at an interval of 0.005Qd, and the experiment was carried out under stable flow rate condition. It shows the flow adjacent to the blade suction side gradually evolved from the flow separation into a broken vortex. The stall vortex moved toward the impeller's inlet and continuously grew, and resulted in significant changes in the main flow direction at the channel inlet. The formation and development of the other vortex structures in channel were closely related to the stall vortex at the inlet. The second method is the dynamic flow rate measurement, and the results show that the stall is not caused by the increase in the relative inflow angle. It was obtained that the velocity value in the stall channel near the suction side rapidly decreased; however in the nonstall channel, the velocity value increased at the channel inlet. By analyzing the velocity distribution in both flow channels before and after the stall, the mechanism of alternating stall is well explained. Meanwhile, it was obtained that the stall was more likely to originate from the flow separation near the blade suction side for low specific speed impeller.