Abstract: For understanding the behavior of constricted arc with flux strip in ultra-narrow gap,metal arc welding tests were carried out on I groove deposited with the flux strips.Results show that the height of lower sidewalls uncovered by flux strips controls the height of sidewalls melted by arc.When the height of lower sidewalls uncovered by flux strips is very small,the arc only heats the bottom of the groove,and the arc length is longer while short circuit ends and the arc reignites.Therefore,the voltage is higher as arc reignites,and then rapidly decreases,meanwhile the current peak of short circuit is larger.When the height of lower sidewalls uncovered by flux strips increases,the heating range of the arc expends from the bottom of the groove to its sidewalls,and the arc length is shorter as the arc reignites.Therefore,the voltage is lower as arc reignites,and then slowly decreases,even doesn't decrease.At the same time,the frequency of short circuit increases,the current peak of short circuit lowers,and the probability of instantaneous short current increases.