Abstract: A new type of swing arc narrow-gap gas metal arc welding torch was used to conduct the narrow-gap horizontal welding test on FH40 thick plates. The relationship between process parameters and backing bead formation, droplet transfer and defects were studied. The results showed that the are swing amplitude influences greatly on the weld width and the surface formation, and the welding speed has the greatest impact on the weld penetration and the asymmetry of the weld. The molten pool could not widely expand in welding without arc-swing, and the weld bead becomes narrow and convex to result in lack-of-sidewall penetration in the weld. In welding with swing arc, the droplet transfer form and the molten pool flow state could be changed. An appropriate swing amplitude was helpful for the molten pool to fill the groove evenly and to obtain a well-formed symmetrical weld with satisfied sidewall penetration. An excessive large amplitude of arc swing would accelerate the sagging of molten pool, even to cause welding defects such as undercut and weld bead convexity.