A new method of welding the ultra-narrow gap with flux strips constraining arc can heat both sides of the root of groove at the same time,and realize the single channel multi-layer welding of 4.5 mm straight groove. We carried out a series of welding tests of ultra-narrow gap welding for horizontal butt joint and realize 60 mm thick butt joint by changing the process parameters. The results show that the welding voltage affects the melting depth of the upper side of the groove,the welding current and welding speed determine the melting amount of the welding wire,and the reasonable cooperation between them can make the minimum downward flow of molten pool. The ultra-narrow gap welding pool is small, the solid-liquid contact surface is relatively large,its surface tension is easy to balance the gravity of the pool,and it has certain advantages to prevent the defects of the pool in the transverse welding.