The invention discloses a flaring device for a core body on a radiator core body assembly machine. The flaring device comprises a plurality of flaring expansion heads; the flaring expansion heads penetrate through each main board installation template and are fixed on each flaring expansion head fixing plate through a combined locking block; each flaring expansion head fixing plate is arranged on main board explorator through flaring expansion head fixing plate sliding rails and flaring expansion head fixing plate sliders, and is pushed to move by flaring expansion head fixing plate air cylinders fixed on the main board explorators; and the tail end of each flaring expansion head fixing plate is provided with two flaring expansion head fixing plate air cylinders. The flaring device disclosed by the invention is simple in structure and convenient to use; the flaring device is arranged on the main board explorators, so that a radiator core body assembly process and a radiating pipe flaring process are performed simultaneously, therefore production efficiency is increased; a set of flaring facilities does not need to be developed additionally, thus reducing production cost and being in favor of large-scale production.