The invention relates to a special clamp for welding a lengthening drill or an expanding drill. As the welding lengthening drill or the expanding drill is welded by adopting a high-speed steel edge part and a medium carbon steel handle part, and no special clamp is adopted, the problems of high restoration work load caused by edge tool deformation and direct scraping owning to serious deformationare caused. A center bracket and a base are connected by an upward-concave groove on the center bracket and a locating key in a downward-concave through groove, two bolt heads connecting a bolt are arranged in each inverted T-shaped through groove of the base, the center bracket is fixedly connected with the base by two connecting bolts, a center base of a center is arranged in a center height adjusting hole on the center bracket, the center presses the bolt and is in threaded connection with the center bracket and the center base, the center base and the center bracket are connected with an adjusting bolt in a bolt penetrating-through hole on the center base through a waist hole passing through the center bracket, and the center heads of two centers are arranged coaxially. The special clamp is used for clamping and fixing in the welding of the lengthening drill or the expanding drill.