<p>A three-dimensional transient model for weld pool dynamitic behavior has been established in swing arc narrow gap gas metal welding (SA-NG-GMAW) with additional wire, taking into account the arc trajectory and joint geometry. The inclination of the arc in the plane perpendicular to the weld and the change in the direction of arc movement is dealt with by the coordinate system transformation. The heat source distribution parameters are determined according to the arc behavior characteristics captured by a high-speed camera system. The major forces acting on the weld pool are considered in the model, including arc pressure, arc plasma drag force, electromagnetic force, surface tension, and Marangoni stress. The temperature distribution and fluid flow within the weld pool are analyzed and the influence of different welding parameters on them is discussed. The results show that there are large vortexes at the bottom of the left and right sides without additional. The liquid metal mainly flows to the side wall and the bottom of the two sides, which is beneficial to the increase in sidewall penetration, but will produce a “double peak” phenomenon. In the case with additional wire, the flow pattern of liquid metal from main welding wire was affected by liquid metal from additional wire and the vortexes on both sides were reduced. Part of the liquid metal flowed toward the middle of the weld pool bottom, and the “double peak” phenomenon was weakened. The “double peak” phenomenon tends to diminish with the reduction of swing frequency. Meanwhile, the width of HAZ outside the weld pool is smaller than that without additional wire, which decreases with enhancing the swing frequency.</p>