Abstract: The narrow gap hot wire TIG welding in vertical position of 12CrNi5MoV thick plate was studied, and the influences of groove forms, groove gap, welding current, wire feeding rate, pulse frequency and swing of tungsten on the formation of weldseam were researched.Resultsshowed that compared with the X shaped groove, the narrow gap U shaped groove reduced the welding filling quantity by 76%; the welding parameters of narrow gap hot wire TIG welding for thick plate were determined, which realized the high efficiency and high quality welding of 150 mm-thick 12CrNi5MoV steel plate.