The invention discloses a composite forming method of gas bulging forming and thermal forming and a device used by the same. The method comprises the following steps of: pre-bending and welding a rough material to form a conical part; placing the lower end of an air bag in a clamping groove below the air bag; casing the rough material on the outer surface of a thermal forming mould; placing an upper cover of the mould above a gas bulging forming mould; hanging a tool part on a work bench of a gas bulging forming furnace; communicating the air bag with an external ventilating pipeline; closing a furnace door; starting a heating device; performing thermal forming; performing gas bulging forming according to a gas bulging forming process when the temperature reaches a gas bulging forming temperature; taking the tool part out after the tool part is cooled; and disassembling parts according to an order reverse to the order for assembling the tool parts. The device used by the method comprises the thermal forming mould, wherein the gas bulging forming mould is arranged above the thermal forming mould; the thermal forming mould is connected with the gas bulging forming mould into a whole; the upper cover of the mould is arranged above the gas bulging forming mould; the air bag is arranged in the clamping groove above the air bag and in the clamping groove below the air bag; and the air bag is communicated with the ventilating pipeline.