Abstract: Abstract: When centrifugal pumps work under small flow rate conditions, the flow separation usually occurs inside. The flow separation often leads to the instability of inner flow, which can make the pump performance worse and maybe cause huge damage to centrifugal pumps. To disclose the development of flow separation, the technique of Particle Image Velocimetry (PIV) was applied to measure the inner flow field in a centrifugal pump. A centrifugal pump, whose specific speed was 73, was selected as the research model. The designed flow rate was 27 m3/h, and the designed head was 11 m. The blade number of the impeller was 6 and the outlet diameter of the impeller was 198 mm. The inlet diameter of the volute was 220 mm and its inlet height was 33 mm. To meet the requirements of the PIV test, the impeller and the volute were all made of plexiglass. The flow fields in the pump under seven different flow conditions were tested and the unsteady PIV test, including 12 different times, was done at 0.2Qd. All the experiments were conducted in an open loop, which consisted of a reservoir open to air, a suction valve, a test pump, a discharge pipe, and a discharge valve. The inner flow test results were analyzed carefully. To quantify the analysis of the test, the angle of flow separation and the circulation intensity were introduced as two analysis parameters in the paper. The definitions of the two analysis parameters were also presented in detail. The test results at different flow rate conditions showed that there was no flow separation in the pump at the design condition. When the flow rate was smaller than 0.6Qd, the flow separation began to occur and the flow separation developed fully under 0.2Qd. With the decrease of flow rate, the separation bubble moved toward the middle position and the outlet of the impeller channel. The test results at different impeller positions indicated that a separation bubble will appear in the impeller channel when the impeller channel gets close to the cut-water of the volute. When the impeller channel gets through the cut-water, the separation bubble develops rapidly. The separation bubble will disappear gradually when the impeller channel is far away from the cut-water. The variation gradient of angle of flow separation near pressure side of the blade is larger than that near suction side. With the decrease of flow rate, the variation gradient of angle of flow separation in the channel increases gradually. The number of separation bubbles can be obtained exactly by quantitative analysis of the angle of flow separation. The quantitative analysis of circulation intensity displays that the effect of rotor-stator interaction on flow separation gets obviously weak after the impeller rotates 135 degrees. The circulation intensities of the three channels near the cut-water of the volute are bigger than that in other channels. The study in this paper shows that the angle of flow separation and the circulation intensity are very appropriate to analyze the flow separation in centrifugal pumps. The research results are useful for the design and optimization of centrifugal pumps, especially for pumps with low specific speed.