Abstract: The droplet transfer behavior and electrical signal of low-current MAG welding were studied by a high-speed camera system and an NI AD data acquisition system. The results show that the droplet transfer mode of low-current MAG welding of thin steel plate is mainly short-circuit transfer. In the short-circuit stage, the voltage drops rapidly, and the current rises rapidly, forming a short-circuit liquid column. When the current increases to a certain value, the liquid bridge bursts; the droplets carry heat and kinetic energy to the molten pool, and the arc voltage returns to the no-load voltage; the arc is re-ignited. According to the different heat source characteristics of the arcing and short-circuit stages in the short-circuit transition, a new MAG heat source model for the low-current short-circuit transition is developed. The double elliptical heat source model with different power is used in the arcing stage, and the homogeneous body heat source model, considering the droplet heat energy and kinetic energy, is used in the short-circuit stage. The heat input of low-current MAG welding is described by double elliptical and homogeneous body heat source periodic loading. At the same time, the effects of MAG welding on the geometric morphology, width, and depth of the molten pool under different heat inputs are studied to further verify the developed heat source model. The results show that the calculated results are in good agreement with the experimental results.