The invention provides a machining method for a plug in a heat transfer pipe of a heat exchanger. The machining method comprises the main machining steps that the size of the plug is measured; a sleeve workblank and a cone core sliding block are manufactured; the cone core sliding block is matched with the sleeve workblank; the sleeve workblank well provided with the cone core sliding block is pressed in a die with a mold cavity, the sleeve workblank deforms because of the extrusion action so as to be attached to the cone core sliding block; and finally, the sleeve workblank is taken out of the mold cavity of the die, and a blind hole, the outer wall and the end of the sleeve workblank are machined, and then the finished plug is obtained. According to the machining method, the manufacturing steps are few, a sleeve is matched with the cone core sliding block mainly through one die, mechanical devices related to the machining method are few, and very high precision calculation is not needed; and in addition, the machining method can adapt to manufacturing and machining of plugs with various diameters, and applicability is high.