In order to study the influence of a novel current waveform control method of ultrasonic-frequency pulse (UFP) on arc behaviour, metal transfer process and the welds formation, monitoring system and image processing algorithms were employed for extracting the welding characters. Mechanics, defect, microstructure and mechanical property analysis based on pulsed GMAW was carried out. The results showed that, compared with the conventional pulsed GMAW, the ultrasonic-frequency pulse not only increased axial arc plasma jet force, arc force and droplet falling acceleration, but also suppressed the welded porosity formation, decelerated the microstructure regional element segregations, refined the grain and increased the microhardness property of welded joint. The study helped to reveal the mechanism for improving welding quality of ultrasonic-frequency pulsed GMAW.