Abstract: Objective The purpose is to develop the heat source model for swing-arc narrow gap GMA welding. Methods This paper considers the influence of arc swing, geometric feature of welded joint, movement of additional wire and weld surface shape on arc heat flux distribution, based on macro thermal transfer theory and geometric feature of weld cross section. By using ANSYS software, the transient temperature profile and thermal cycle curve in swing-arc narrow gap GMA welding are calculated and their distribution features are analyzed. Results The results show that the established heat source model can reflect the moving path of swing-arc and its thermal action feature, the calculated geometry and size of weld cross section and thermal cycles of temperature measurement characteristic point agree well with the experimental data. Conclusion It is proved that the model can truly reflect moving track and distribution characteristics of swing-arc with additional wires, and has high accuracy. There is an obvious “double peak” feature at the bottom of the cross section of the weld without additional wire, while the “double peak” feature is diminished with additional wire. The width of HAZ slightly increases when the main and additional wires are synchronously swing.