In this paper, a novel swing arc welding system developed for horizontal narrow gap gas metal arc welding (NG-GMAW) which was used in welding and joining heavy plate structures was designed and attempted. Using this welding system, two sound heavy plate beads, i.e., “multi-layer single-pass” welding and “multi-layer double-pass” welding structures, were obtained through adjusting swing amplitude and frequency. Two different welding structures are both without sagged molten pool and interlayer slags, especially obtaining the complete sidewall penetration. All these welding features ensure the bead with higher tensile properties and hardness than the base metal, respectively. This welding system displays considerable flexibility and convenience and possesses a tremendous advantage in the auto-welding industry.