The invention discloses a method for preheating before friction stir welding, which comprises the following steps of: S1, assembling a disc surface and a tire ring which are pre-welded together to form an interference fit rim; s2, the assembled interference fit rim is moved to a welding tool to be fixed; s3, preheating at a welding bead; s4, determining the surface temperature of the weld bead by using a laser thermometer, and completing preheating after the surface temperature reaches 200-250 DEG C; and S5, after preheating is completed, friction stir welding is conducted immediately. By means of preheating assistance, the welding heat input amount can be increased, welding materials can be softened, the requirement for downward pressure is lowered, meanwhile, the welding difficulty can be lowered, abrasion of the stirring head is reduced, the service life of the stirring head is prolonged, the welding axial force and the welding difficulty of high-melting-point metal are lowered, high-speed welding is easy to achieve, and the welding efficiency is improved. And in addition, a proper preheating temperature is adopted, so that the material generates good plastic flow, and the generation of defects such as tunnel holes is avoided.