Abstract: The droplet transition behavior of real-time welding process of GMAW swing arc vertical welding was conducted using high-speed video system when gap existed and changed. Analysis of the effect of the droplet transition behavior on the quality of arc are analyzed, and the combination welding process parameters of the current, arc voltage probability curve interpretation collected by arc welding quality analyzer, experimental results have been verified and supplementary explained. Consequently, the mathematical model of the GMAW vertical-welding has been established using the finite element numerical simulation technology. The transient temperature field been verified and analyzed. The results show that, with the increase gap of size (range from 0 to 4 mm), the droplet transition shape of GMAW vertical welding changes from short circuit to special droplet, and the simulation results show that the GMAW vertical welding heat affected zone has larger temperature gradient characteristics.