The internal flow field of hydrodynamic coupling was tested by PIV technology. On the braking condition (i=0), flow velocimetry on radial section of turbine was extracted based on PIV cross-correlation algorithm of two continuous images, visualization and quantitative measurement of internal flow field in hydrodynamic coupling were achieved. Flow structure and velocity distribution of internal flow field in turbine on the braking condition were studied. Reasons caused the reverse flow and secondary flow were analyzed specifically. The results showed that the phenomenon above could be reduced by designing the internal structure of hydrodynamic coupling. It could reduce energy loss and raise its work performance. 