<p>Hairpin vortices and vortex packets are the most typical coherent structures in turbulent flows. Studying their generation and evolution mechanisms is key to understanding the essence of turbulence. Single hairpin vortices in a laminar boundary layer are generated by a synthetic jet, and their formation and evolution are investigated in this study. The results show that during the formation of a secondary hairpin vortex, a local high-shear region is induced below its head. Both the wall-normal position of this high-shear region and the velocity gradient at the onset of secondary vortex formation lie within a specific range. And in this process, a local high-shear region is induced below the head of the secondary vortex. This mechanism has also been validated in the turbulent boundary layer, thereby confirming its universality. Besides, the increase in inflow velocities leads to stronger shear during the formation of secondary hairpin vortices and causes their initiation to occur increasingly higher above the wall. This study contributes to the formation conditions and quantitative criteria of secondary hairpin vortices and provides a foundation for future research on vortex evolution in the turbulent boundary layer.</p> Graphical Abstract <p></p>