A continuous feeding flux strips constrained arc ultra-narrow gap welding method was developed. The process test was carried out by changing the thickness of flux sheet and the feeding speed of flux strip, and the heating characteristics of the arc under various parameters were analyzed. The results show that increasing the thickness of flux sheet and the feeding speed of flux strips can enhance the restraint effect on the arc,increase the melting width of the weld root and reduce the melting height of the side wall; Increasing the melting coefficient of the flux sheet will increase the constrained height of the fixed wall of the melted flux sheet, and the height of the arc burning on the side wall will decrease, so that the arc energy density increases, which is more conducive to the arc heating the root of the side wall. When the melting coefficient of the flux sheet is less than 0.31, the constrained height of the fixed wall of the melted flux sheet is smaller, and the heating effect of the arc on the side wall enhances.