Gas metal arc welding (GMAW) using an alternating current waveform is usually termed as variable polarity GMAW (VP-GMAW), during which the electrode polarity switches between positive and negative periodically. The arc properties and the droplet transfer in VP-GMAW would be different from traditional direct current GMAW. In order to clarify the droplet transfer phenomena during a VP-GMAW process, a unified numerical model including the interaction between the arc plasma and the moving droplet is developed. The simulation results indicate that the arc plasma generated at electrode negative polarity shows a less constricted shape, a lower plasma temperature and velocity, compared to positive polarity. The resultant droplet is found to have a bigger size and a lower temperature than that of direct current gas metal arc welding with the same average welding current. Moreover, a quantitative analysis of the heat fluxes into the electrode is further conducted to explain the thermal mechanism for the differences in droplet properties between variable polarity and direct current gas metal arc welding. Finally, the simulated results are compared with the high-speed images, and the simulated and experiment results show good agreements. (C) 2017 Elsevier Ltd. All rights reserved.