Abstract: In the welding of ultra-narrow gaps with a groove width of approximately 5 mm, flux bands were used to confine the arc at the bottom of the groove, effectively improving the fusion effect of the groove’s side wall, and the flux bands have played a key role in this process. A U-shaped thick flux band was designed. The thickness of the two side walls of the flux band was increased to 1.1 mm. The gap distance between the welding wire and the flux band was eliminated, and their relative positions were fixed. The stability of the flux band in confining the arc was successfully improved. The process characteristics of the arc ultra-narrow gap welding method constrained by thick flux bands were studied through experiments. The results show that the welding voltage and the feeding speed of the thick flux band jointly determine the acting length of the thick flux band on the arc. Only when the acting length of the thick flux band is appropriate can the arc uniformly heat the groove in all directions, ensuring the good fusion of the side wall and the root of the side wall. Furthermore, on the premise of ensuring the appropriate acting length of the thick flux band, choosing larger process parameters is conducive to improving the fusion effect of the side wall of the groove and can obtain a relatively stable droplet transition form, finally completing the single-pass multi-layer welding with an ultra-narrow gap.