Aiming to analyze the flow field characteristics of the vortex separator in the circulating biological flocculation system, the test scale vortex separator was taken as the research object. The noncontact flow field test technique, particle image velocimetry (PIV) technology, was used to measure the flow field in the test scale vortex separator and analyze different hydraulic powers of vortex separator. The distribution of velocity, velocity and vorticity in the flow field of cyclone separators (248 s, 83 s and 49 s) under residence time condition was analyzed. The results showed that under different HRT conditions, the lower left corner and upper region of the inner region of the vortex separator showed a certain vortex. At the same time, with the acceleration of the HRT, the velocity direction in the inner sleeve was approximately the same, only a small two flow was produced near the wall of the cylinder, and the velocity direction of the particle in the silo was also tending to be the same. Although HRT was accelerated, the axial and radial velocity was changed little, and the proportion of different size and velocity was basically the same; the clockwise and clockwise vortices were basically the same under different conditions, and the slower the HRT was, the smaller the vorticity of the flow field was, and the higher vorticity distribution tended to be uniform with the fast vorticity distribution of the HRT, that was, the high vorticity. The region was gradually increased; the PIV test limited penetration ability because of the certain laser energy. Therefore, the results of the complex structure to the PIV test still needed to be improved. The research results had important reference value for improving the structure of vortex separator and selecting the best working condition.