Abstract: When flux strips applied to constrict arc have different relative positions with arc in ultra-narrow gap,flux strips can cause different constricting effects on arc,so the position of flux strips in the arc zone is the linchpin of constricting arc in time of ultra-narrow gap welding with flux strips constricting arc.By analyzing the position of flux strips in the arc zone,the results indicate only when three positional conditions are satisfied,flux strips can efficiently constrict arc,firstly flux strips need cling to sidewalls in the direction of radial of arc;secondly flux strips need proper action length in the direction of arc length;thirdly the tip of wire needs be located in the center of flux strips.Furthermore,under arc current exceeds 240 A,flux strips can easily cling to the surface of wire because of action of electromagnetic force,the position of flux strips in the direction of radial of arc can be located efficiently by supporting flux strips.