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A centrifugal pump with specific speed of 74 was studied. Hydraulic performance test was done firstly, and the result showed that the head changed little with the flow reduced when flow was less than 18m3/h. In order to research the flow instabilities under the above H—Q curve, PIV technique was adopted to discover the onset and development rule of unstable vortex within the impeller. The results showed that the flow was separated at 0.6QBEP, developed at 0.4QBEP and spread almost the whole impeller passage at 0.1QBEP. In addition, flow in the passage near the tongue was the most unstable, and the vortex was also occurred in this passage firstly by comparing with rest of passages. With the flow rate decreasing, the vortex on the pressure side was increased and converged to a large-size vortex which spread to the outlet of passage and moved to the centre area of the passage in the end.