The invention belongs to the field of machines and particularly relates to an expanding cone applied to mechanical expanding of a longitudinal welded pipe production line. The expanding cone comprises a conical cone body. A connecting hole and a lubricating oil hole are machined in the big-head end of the cone body. The expanding cone is characterized in that a plurality of rectangular grooves are machined in the outer surface of the cone body, the rectangular grooves are distributed evenly and are the same in width and height in the longitudinal direction, and rhombic conical convex strips are formed between the rectangular grooves of the outer surface of the cone body; each rhombic conical convex strip is provided with two rectangular abrasion-proof strips through countersunk bolts; one side edge of each abrasion-proof strip extends to the position above the corresponding rectangular groove, and consequently each rectangular groove and the corresponding abrasion-proof strip form a longitudinal guide key slot with the T-shaped section in a surrounding mode. The expanding cone solves the problems that machining is difficult, the period is long, material waste is severe and production cost is high in the prior art, and is simple in structure, convenient to manufacture, low in production cost, short in manufacturing period and convenient to repair.