The invention discloses a silicone rubber air bag forming method for manufacturing of a heat insulating layer of an engine. The silicone rubber air bag forming method comprises the steps that a die and a corresponding raw silicone rubber piece are prepared according to requirements of an air bag; then vulcanization, adhesion and secondary vulcanization are conducted on a straight cylindrical section of a material, vulcanization is also conducted on an end part of the material, the two ends of the straight cylindrical section sequentially adhere to the end part after the straight cylindrical section is formed through adhesion, and the formed silicone rubber air bag is obtained after vulcanization. The silicone rubber air bag product manufactured through the method is stable in quality, high in surface dimension precision and uniform in thickness; the air bag is evenly expanded after being inflated in use, and the stress condition of the heat insulation layer is improved obviously; the problem of insufficient vulcanization of the inner surface of the silicone rubber air bag is completely solved, and the performance of rubber materials is brought into full play; the actually-measured performance of the product is basically the same as that of the rubber materials, and the service life is greatly prolonged.