Abstract: Constricting arc with flux strips is employed in ultra- narrow gap welding, by measuring the cross sectional sizes of welds under different welding parameters, heating characteristic of constricted arc was analyzed based on the variation of the cross sectional sizes of welds.The results indicate the arc with effective constriction in the ultra-narrow gap benefits from preventing itself climbing up along sidewalls, and ensure a reliable fusion of sidewalls;arc morphology is the main factor to decide fusion of sidewalls, the increase of welding current or feeding rate of flux strip can make the heating position of arc move down, the heating height of sidewalls decrease, energy density of arc enhance, which more benefits from achiving good fusion of sidewall roots;the increase of voltage can make the range of cathode spot moving expand, which can enhance the heating effect of arc column and cathode spot on sidewall roots; the reasonable matching of voltage, current and feeding rate of flux strip is good to obtain proper arc morphology, and makes arc have a effective heating in three directions of ultra-narrow gap.