Hydrocyclone has been extensively used in various separation processes. For example, in the sewage source heat pump system, a De-Foulant Hydrocyclone with Reflux Ejector (DFHRE) can fully use the kinetic energy of its overflow to suck its underflow and flush away the foulants flowing out with the underflow. Our previous experiments in 2018 showed that the reflux ejector could prevent the underflow orifice from blocking, and reducing the suction angle could increase the separation efficiency by approximately 10% whereas decreasing its energy consumption markedly. To date, however, its mechanism is still unclear. Therefore, in this study, we used the Particle Image Velocimetry (PIV) method to study the flow field in DFHREs with different suction angles and compared it with that in the traditional hydrocyclone without reflux ejector. Results indicated that, in all the tested DFHREs, there was a negative-axial-velocity area in the center of hydrocyclones, which was not found in the center of the tested traditional hydrocyclone. The negative axial velocity produced by the overflow-suck-underflow decreased with increasing suction angle whereas increased with increasing inlet velocity. The split ratio and total pressure drop of the DFHRE are respectively and approximately 18.5% and 28.3 kPa greater than those of the traditional hydrocyclones at the suction angle of 30°. This proved that the experimental performance of the sewage suction ejector used in hydrocyclone, that is, the suction effect of the reflux ejector and the separation efficiency of DFHRE were the best at the suction angle of 30°. Besides, the obtained change of flow field caused by the reflux ejector provided a new idea for enhancing hydrocyclone separation.