Abstract: As for the low impact toughness of weld metal for large hydro-turbine runner, effect of welding methods and gas content on impact toughness of ultra-low carbon martensitic stainless steel welding wire deposited metal is investigated. The results show that the Laser-MIG hybrid welding in addition to TIG welding can greatly improve the impact toughness. TIG can be used for weld repairs of stainless steel runner. Laser-MIG welding could promete the using in the group welding of laminas, vanes and band. The impact toughness decreased with the increase of C, O and N content of deposited metal, and O is the major factor. Spherical oxide inclusion can generate crack in deposited metal with high O condition, and decrease crack initiation and expansion energy.