Mold for partially shortening circumference of tube billet and using method of mold

A mold for partially shortening the circumference of a tube billet, comprising an upper mold, a lower mold and a mold core used for being inserted into a tube cavity of the tube billet, the upper mold being provided with an upper mold cavity, the lower mold being provided with a lower mold cavity, and the upper mold cavity and the lower mold cavity being matched to form a mold cavity, wherein after the mold core is placed in the mold cavity, a tube billet deformation cavity is formed in the mold cavity, and an outer contour of the mold core is configured to be deviated inwards by a distance of t+a compared with inner surfaces of the upper mold and the lower mold, wherein t is the wall thickness of an initial tube billet that is selected and a is larger than 0.05 mm and smaller than 10 mm;

• and the upper mold cavity and/or the lower mold cavity are/is in the shape of a flaring, the two sides of the flaring are arc-shaped, and included angles are formed between the two sides of the flaring and the moving direction of the upper mold and/or lower mold.