The invention discloses an on-line arcuation device of a bumper, which is used for solving the problem of bumpers with small processing arcuation radius. The technical scheme is as follows: the on-line arcuation device comprises a bracket, a form block and a core, wherein a cover plate on the upper part of the bracket is provided with two rows of adjusting screw rods, and each adjusting screw rod is provided with a sliding block in screw-thread fit with the adjusting screw rod; two ends of the form block are in running fit with the axle hole of the sliding block; the form block is a form working face along the axial middle part; the cross section of the form working face is a profiled hole matched with the profile of the cross section of the bumper; the bumper passes through the profiled hole; the core is arranged in the bumper; and the cross section of the form working face is the a special-shaped hole matched with the profile of the cross section of the bumper. The invention has small arcuation resistance, a great quantity of force application points and favourable form surface quality, can finish product arcuation which has complex cross section shape and radius of 1800 mm on line.