With the increasing of the single unit capacity of pumped storage unit, the steel plates for the key parts of the unit are becoming thicker and thicker. In order to realize the robot welding of the large-scale pumped storage ring and reduce the labor intensity, the welding structure of the fixed guide vane of the seat ring is innovatively designed, and the complicated irregular welding bead is changed into a simple straight welding bead, through the design and parameter test of L type narrow gap MAG welding gun system suitable for T-joint narrow gap groove welding, and the integration of robot narrow gap welding system, a set of mature multi-robot narrow gap MAG welding technology has been developed and successfully applied to the welding of large pumped storage seat rings, which improves the efficiency and quality reliability of the seat ring robot welding, the labor intensity of the girth welding of the large-scale pumped storage sty ring is reduced.