Abstract: Baihetan 1 000 MW hydro unit is all welded with domestic HS13/5L welding wire, which has successfully achieved a major breakthrough in China's high-end equipment manufacturing. HS13/5L welding wire is the welding material identical with the hydraulic turbine runner, belonging to 13Cr supermartensitic stainless steel. However, the toughness of the deposited metal in metal active gas arc (MAG) welding is lower than that of the base metal. In view of this problem, the welding process is adjusted from flat to vertical-up welding position, to improve the impact toughness of MAG welding deposited metal. And the microstructure and impact properties of the deposited metal are compared and analyzed. The results show that the room temperature impact absorption energy of the as-tempered deposited metal in the vertical-up welding position reaches more than 120 J, which is about 40% higher than the flat welding position. There is no obvious difference in the phase composition of the deposited metal microstructure between the two welding positions, the as-welded microstructure is composed of quenched martensite, retained austenite and delta ferrite and the as-tempered microstructure is tempered lath martensite, reversed austenite and delta ferrite. The density of oxide inclusions in the vertical-up welding deposited metal is about 22% lower than the flat welding position. The impact fracture of the deposited metal shows the characteristics of ductile fracture for both welding composition as a whole, and the toughness of the deposited metal in the vertical-up welding position is better than that in the flat welding position.