The particle image velocimetry technology, separation performance experiment and numerical simulation methods of separation performance were applied to analyze the separation performance of three kinds of hydrocyclones. The method for evaluating the performance of oil?water two-phase separation in a horizontal circular pipe was established. The results showed that the helix separator and the diversion vane separator can form axial reflux in a axial central region of the flow field, and there were different locus of zero vertical velocity in the two structures. When the inlet Reynolds number was 6.68×104, although the diversion vane separator formed a rotating flow with the maximum tangential velocity, the attenuation rate of tangential velocity was also the largest, and the value of maximum tangential velocity attenuation rate within the 500 mm length was 94.05%. The oil core formed by the helix separator had a better degree of aggregation with the maximum oil volume fraction at the axis of the pipe. In order to quantitatively evaluate the performance of separators with different structures, the calculation method of oil?water separation efficiency in circular pipe was established by taking the concentration of oil core and the extension length of oil core in the pipe. By comparing the separation efficiency of the three structures under different Reynolds numbers, the helix separator showed the best separation performance. When the inlet Reynolds number increased in the range of 4.14×104~10.7×104, the oil?water separation efficiency of the helix separator increased from 16.6% to 82.1%, which was higher than those of the other two structures. The evaluation results of oil?water separation efficiency showed the consistent regularity as the experimental results, which verified the accuracy of the proposed separation efficiency evaluation method.