Vaporizing Foil Actuator Welding (VFAW) has the promising potential to be widely used to join steel with non-ferrous metals, such as titanium, magnesium, and aluminum alloys, which is difficult with fusion welding techniques. The aluminum foil acts as the medium of energy transmission from electric energy to dynamic energy during VFAW process. Its working efficiency is of vital importance for the welding technique. In this article, the influence of the working efficiency of the aluminum foil on impact velocity between the flyer and target was explored via Photon Doppler Velocimetry (PDV) experiments. The experiments were carried out with foils of three different thicknesses under varied input energies. When the distance between the fly and target was 2.5mm with the impact angle of 5°, the vaporization energy of the foil is the key factor contributing to the impact velocity. According to the analysis of impact velocities, the 0.051mm –thick aluminum foil has the best efficiency with the input energy under 3 kJ, 0.076mm-thick aluminum foil works most efficiently with the input energy of 3kJ to 6kJ, and with the input energy of 6kJ to 12kJ, the 0.127mm-thick aluminum foil has the highest working efficiency.