Abstract: Abstract: Axial vortex is a kind of special relative flow within the adjacent blades of hydrodynamic coupler. The distribution of flow field structure and its evolution law are considered as the important foundation for the research on the internal energy losses of hydrodynamic coupler. Transparent square cavity hydrodynamic coupler model prototype made by plexiglass was taken as the experimental measurement object. Axial vortex flow images of hydrodynamic coupler under braking condition were collected based on particle image velocimetry. In order to improve the quality of flow images, image preprocessing was used to identify the flow characteristics of particles in flow images. In order to get a better image calibration result, a special geometric structure of hydrodynamic coupler was taken as an external calibration object. The calibration images were acquired with high-definition CCD (charge coupled device) camera, and then optical characteristics of the special geometry structure were identified and extracted by image processing technology. Dynamic calibration coefficient of flow images was obtained finally. The flow field of axial cross section in pump was identified and extracted based on straight line detection algorithm of Hough transform, and the linear characteristics of particle trajectories were identified and extracted successfully. The flow field of axial cross section in turbine was calculated based on cross-correlation algorithm between consecutive 2 frames of particle images, and the technology of interrogation window offset was used to extract the velocity field and vorticity field. In order to identify and eliminate the incorrect flow velocity vectors of the initial calculation results, a method of setting a threshold was used to check the difference between ambient velocity vector and the center velocity vector, and the consistency and coherence characteristics of flow velocity vectors on the flow area were detected by this method. Subsequently, calculation results of flow field were optimized, and a much smoother flow pattern was acquired finally. On the basis of the flow field measurement results, the unsteady viscous flow phenomena of hydrodynamic coupler under braking condition were researched, and the distribution law of axial cross section was analyzed in detail. Research results indicated that: Firstly, the flow of axial cross section in pump was a compound accelerated motion; secondly, there were a lot of multi-scale vortices in the main flow region of axial cross section in turbine, and the flow velocity in the mainstream area was 0.2-0.4 m/s. There were many small-scale vortices in the intersection area between outer ring and blades, and the flow velocity in these area was 0.6-1.1 m/s. In addition, some small-scale vortices which existed in the intersection area between upper blade and interface area were different from others; the flow direction of these vortices was the same as the mainstream circulation direction, the vorticity value of this area was -8 s?1, and these small-scale vortices would promote transmission and conversion of hydrodynamic energy. On the contrary, the flow direction of small-scale vortices in other areas was opposite to the mainstream circulation direction, and the vorticity value in these areas was 13, 15 and 20 s?1. The small-scale vortices in the local region were mixed with the main circulating vortex in the main region, and as the result, the internal flow became slow with a large amount of energy loss. The phenomena of irregular secondary flow and reverse flow came into view near the middle flow area of blade. According to the visualization results of axial vortex flow in hydrodynamic coupler under braking condition, the reasons to the generation about axial vortex in hydrodynamic coupler were explored, and the influences of flow energy transfer and dissipation were analyzed. The result will provide valuable reference for the study of axial vortex flow phenomenon.