Abstract: To understand the characteristics of base metal melting and mechanism of pool formation in ultra-narrow gap with constricted arc by flux strip,metal arc welding tests were carried out on I groove deposited with the flux strips.By using of method for interrupting arc quickly,the melting forms of flux strips,groove's bottom and sidewalls in the welding process were hold.Results showed that,the arc was thermally constricted due to melting flux strips,in addition,the arc got a wall constriction as it contacted with the non-molten flux strips.These constrictions could control the heating area of the arc in narrow gap.While the height of lower sidewalls uncovered by flux strips was proper,heating area of the arc was constricted within groove's bottom and the roots of sidewalls,filler metal droplet impingement and arc pressure were concentrated on the front region of molten pool.As a result,these factors caused the flow of molten weld metal,which facilitated expelling the gas and the slag in the molten metal,and welds without the defect could be obtained.When the height of lower sidewalls uncovered by flux strips increased to a certain value,the heating area of the arc extended to higher position of groove's sidewalls,and filler metal droplet impingement was scattered on the groove's bottom and sidewalls.In this case,the flow of molten weld metal could not be formed,so that the gas and the slag could not be expelled in the molten pool,which made welds contain a lot of pores.