Abstract: Key hole TIG welding experiments of 14 mm and 26 mm thick TC4 plates were conducted. The influence of welding parameters on the weld appearance, inner quality, microstructure and mechanical properties of the welded joints was investigated. It has been verified that one-side welding both-side formation of 14 mm thick TC4 plate without bevel preparation can be successfully obtained by using optimized welding parameters.Martensite transformation occurs in the weld and heat affected zone, which leads to obvious increase of hardness compared with the base metal.For 26 mm thick Y-groove plate, 14 mm and 10 mm blunt edge penetration with good weld appearance and inner quality can be achieved by using key hole TIG welding from back side and front side, respectively and conventional TIG welding can be used for filling and covering directly.The strength of the joints of the medium, large thick TC4 plates is equal to that of the base metal, the process performance is good. The Charpy impact toughness of the weld zone is equivalent to that of the base material, while the impact toughness of the heat affected zone is better than that of the base material.