Abstract Pulse current technology is employed to adjust the mass transfer and improve the welding quality, and effects of the pulse frequency (f), load duty cycle (D) and peak current (Ip) on the droplet were analyzed based on Taguchi method during underwater flux-cored arc welding (FCAW). In underwater wet pulse FCAW (FCAW-P), the mass transfer process is observed and the interaction mechanism of the pulse current on the droplet is investigated using in-situ X-ray imaging method. Within one pulse period, the decreasing repulsive force and the increasing gravitational assist the droplets detach from the wire or touch with the welding pool at the end of the peak pulse duration and reduce the droplet detachment angle, improving the mass transfer stability and thus increasing the arc stability and ameliorating the bead appearance. The stable mass transfer process and welding arc increase the effective heat input, and thus decreases the acicular ferrite and increases the pro-eutectoid ferrite in the bead. The hardness distribution and the tensile strength of the beads deposited in conventional and pulse current underwater wet FCAW are identical basically, demonstrating the application of pulse current technology does not deteriorate the mechanical properties.