The invention discloses a pre-forming device of automobile external star wheel by isothermal extrusion. The forming device comprises a punch and a female die, the punch and the female die are matched with a lateral fixing and a vertical shift by a guide pillar. The female die is provided with a die cavity, and a heating device is arranged at the periphery of the die cavity. The heating unit is arranged at the die cavity of the invention to maintain the outer layer temperature of the mold. The invention has the advantages of greatly reducing the geothermal gradient of blank and mold, effectively reducing the thermal dispersion of blank in the forming process, achieving the extrusion pre-forming under the isothermal condition, avoiding the situation of not full die cavity caused by too fast temperature drop, material deformation and persistence increase, improving the qualified rate of finished products, and reducing waste.