Abstract: The hairpin vortices structure induced by a cubic disturbance with a frequency of 2.1 Hz are investigated by time-resolved particle image velocimetry (TR PIV) from both the Euler and Lagrangian view. Firstly, the hairpin vortices' heads could be clearly distinguished form the side-view database, which also indicates that the hairpin heads lift up when convecting down streamwise. Then, the finite-time lyapunov exponents (FTLE) method is utilized to describe the lagrangian hairpin vortices induced by the cubic. It could be seen that FTLE clearly shows both the projection of the hairpin heads and the hairpin necks in the streamwise and spanwise plane. Furthermore, two-point correlation of FTLE at different reference positions along the streamwise indicate that the hairpin vortices statistically with an increasing inclined angle with the flat plate while they are moving downstream.