Based on fluid dynamics theory and electromagnetic theory, a three-dimensional mathematical model is developed describing arc plasma behavior in double-wire gas metal arc welding (GMAW). With fluid dynamic software Fluent, the model is solved. The effect of welding current on arc plasma behavior is studied and the distribution of some physical quantities are completely investigated including arc temperature, current density, heat flux, magnetic flux density. The results show that the arc plasma shape changed significantly, depending on welding current. It is found that with welding current increasing, the arc inclined degree, the maximum arc temperature, peak of heat flux and magnetic flux density on base metal surface increase, and the distribution of current density and heat flux changes from double-peak to one-peak. In addition, as welding current increases, the distribution of magnetic field changes from isolated double magnetic fields to a coupling magnetic field. To validate the accuracy of simulated results, welding experiments are carried out. Arc plasma behavior is monitored by high-speed photography and arc plasma temperature is obtained by spectral measurement. Simulated results are compared with experimental ones and there is a good agreement. The research lays a theoretical foundation for properly selecting welding current in double-wire GMAW. © 2016 Journal of Mechanical Engineering.