Abstract: In the ultra-narrow gap welding of the fed-in flux sheet constrained arc, the interaction between the continuously fed flux sheet and the arc affected the stability of the welding process. A method of rapidly interrupting the arc was used to obtain the shape of the molten pool and the instantaneous melting morphology of the flux sheet at the arc-extinguishing position . The relationship between the action length of the flux sheet on the arc and the degree of arc confinement was analyzed. The results showed that the feeding speed of the flux sheet was increased or the welding voltage was reduced , which could make the action length of the flux sheet on the arc increase. When it increased to a certain extent, the arc was effectively constrained at the bottom of the groove to uniformly heat the root of the side wall, and the flat weld with good root fusion was obtained. When the degree of restraint was weakened, the arc heating area was concentrated on the two side walls, and it was too difficult to form an effective molten pool at the root of the groove.Finally a hole weld was formed.