The invention discloses a hot-setting backing welding flux and a preparation method thereof, and belongs to the field of welding materials for union melt welding. The flux comprises the following components by mass percentage: 18% of aluminum oxide, 22% of magnesite, 5% of natural wollastonite, 9% of fluorite, 2% of silicon mud, 3% of zircon sand, 30% of reduced iron powder, 2% rutile, 3% of ferromanganese, 3% of silicon iron and 3% of ferro-aluminium. The welding flux is spread on the surface of a copper plate uniformly before welding; when the front is welded, the back of a root molten pool and the welding flux conduct partial metallurgical reaction, and the welding flux is heated and solidified to become a backing to assist in back welded joint forming simultaneously; the welding flux takes MgO, CaO, SiO2, TiO2, CaF2 and Al2O3 as basic slagging substances; and the welded joint quality is good. According to the preparation method, a hot-setting agent is added by a special fabrication mode, so that the welding flux forms solid blocks at 200-300 DEG C, no air hole is formed in the welded joint, and the welded joint quality is high.