Abstract: The testing method of three-dimensional particle image velocimeter was introduced. The locating technique of tracer particles was researched and applied to analyze the internal flow rate of centrifugal impeller. Through the experiment, changes of radial velocity component, tangential velocity component, velocity vector of the radial flow and the distribution of secondary flow with different radius were obtained. The test results showed that with increasing the radius, velocity of the pressure side changed from low to high, while velocity of the suction side changed from high to low, and then trended to be steady. The test results verified that the three-dimensional PIV measuring technique had excellent effect as being used to measure inner flow rate of the centrifugal impeller.