The formula of the flameless automatic food heating agent comprises 1 to 5 parts of aluminum powder, 3 to 10 parts of carbon powder, 5 to 25 parts of magnesium powder, 10 to 25 parts of iron powder and 1 to 5 parts of anticoagulant. The preparing of the flameless automatic food heating agent bags (3) comprises the following steps: placing two pieces of food heating agent bags (3) in two sides of a sealed food packaging (2) before putting into a water-proof heating bag (1); adding water into the water-proof heating bag (1) to let food heating agent bags (3) to absorb water; bending an opening side of the water-proof heating bag (1) to seal the opening side; vaporizing water in the water-proof heating bag (1); placing the water-proof heating bag (1) for 10 to 15 minutes; heating food in the sealed food packaging (2) to 60° C. to 80° C.