Abstract: In order to study the vortex-induced vibration response and lock-in characteristics of NACA 0009 hydrofoil under the inlet velocity changing linearly and continuously, a measurement experiment was arranged based on a vortex-induced vibration synchronous measurement experiment platform. Adopting a feedback control method for incoming flow velocity, the small deformation of hydrofoil trailing edge displacement and wake structures were accurately measured, discovering a series of new lock-in phenomena innovatively. The experiment results show that an “amplitude jump” can occur on the hydrofoil when entering and exiting the lock-in regime, increasing hundredfold of the structural vibration amplitude. There is a “frequency increase” phenomenon in the lock-in regime due to the structural natural frequency increasing slowly with the inlet speed. Analyzing the interaction among of vortex evolution, displacement and vibration velocity in the lock-in regime, the experimental study can provide more bases for the lock-in mechanism and fluid-structure interaction characteristics of the hydrofoil.