Abstract: In the welding of ultra-narrow gaps with a groove width of approximately 5mm, using flux sheets to confine the arc at the bottom of the groove can effectively improve the fusion effect of the groove side wall, and the flux sheets play a key role in this process. A U-shaped thick flux sheet was designed. The thickness of the two side walls of the flux sheet was increased to 1.1 mm. The gap distance between the welding wire and the flux sheet was eliminated, and their relative positions were fixed. The stability of the flux sheet in confining the arc was successfully improved. The technological characteristics of the arc ultra-narrow gap welding method constrained by thick flux sheets were studied through experiments. The results show that the welding voltage and the feeding speed of the thick flux sheet jointly determine the acting length of the thick flux sheet on the arc. Only when the acting length of the thick flux sheet is appropriate can the arc uniformly heat the bevel 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 action length of the thick flux sheet, choosing larger process parameters is not only conducive to improving the fusion effect of the side wall of the groove, but also can obtain a relatively stable droplet transition form, and finally complete the single-pass multi-layer welding with an ultra-narrow gap.