Abstract: A pipe hydrocyclone is easy to disassemble in agricultural machinery. Spiral flow can be generated to drive the movement of impurities in the pipe, thereby effectively reducing the accumulation of impurities. Different structural parameters can form differences in the annular gap space of hydrocyclones. The structure of the annular gap space can also dominate the flow field distribution and swirl efficiency. Therefore, this article aims to explore the annular gap spiral flow formed by the hydrocyclones with different structural parameters using physical model experiments. The research results show that the structural parameters of hydrocyclone were dominated by the magnitude and dispersion of axial, radial and circumferential flow rates, where the most contribution was the radial and circumferential flow rates. There was no change in the overall distribution pattern of the annular gap flow field. Furthermore, there was the roughly same variation trend of axial average velocity and standard deviation along the water flow direction under different structural parameters. Both showed a gradually decreasing trend along the way. However there was a relatively small amplitude of the decrease in the axial average velocity along the way. The standard deviation of the cross-section gradually decreased in the constant diameter of the hydrocyclone, as the length increased, whereas, the axial average velocity increased. However, there were relatively small changes in the standard deviation and axial average velocity. Both axial average flow velocity and standard deviation increased in the constant length of the hydrocyclone, as the diameter increased, whereas, there was a relatively large variation in their amplitude. The radial average velocity and standard deviation showed a "V" shaped trend of decreasing first and then increasing along the flow under different structural parameters. The radial average velocity and flow velocity fluctuation of each section gradually increased in the constant diameter of hydrocyclone, as the length increased, but the increase was relatively small. The radial average velocity decreased in the constant length of hydrocyclone, as the diameter increased, but the flow velocity fluctuation of each section increased gradually. The circumferential average velocity and standard deviation of different structural parameters showed a gradually increasing trend along the water flow, but their growth rate in the first half of the annular gap was relatively small. The circumferential average velocity and flow velocity fluctuation showed a gradually increasing trend in the constant diameter of hydrocyclone, as the length increased, but the increased amplitude of both was relatively small. The circumferential average velocity and flow velocity fluctuation showed a gradually increasing trend in the constant length of hydrocyclone, as the diameter increased. Both energy consumption and rotation efficiency increased significantly, as the length or diameter of hydrocyclone increased. In addition, there was a higher increase rate with the diameter than that with the length. This experiment demonstrated that the hydrocyclone with a structural parameter of 70 mm×100 mm presented the highest starting efficiency. The optimal structural parameter of hydrocyclone was in the size of 70 mm×100 mm. This finding can provide a strong theoretical basis and reference for the selection and structural optimization of the hydrocyclone.