Disclosed therein are an apparatus for and a method of automatically welding a flange to a guide thimble tube in a nuclear fuel assembly. The automatic welding apparatus includes: a welding part located on an axially conveying line of a tube for welding a welded surface of the tube inserted thereinto and a welded surface of a flange; a flange supplying part for supplying the flange at a tube inlet of the welding part onto the axially conveying line; and a conveying part mounted on the axially conveying line in such a way as to move the tube and the flange on the axially conveying line, so that they are inserted into and drawn from the welding part.