An airtight device for an airtight air bag module comprises a frame and a cover. The frame comprises at least one wall which limits the expanding track and an opening of an air bag, and the cover is hinged to the frame through an area provided with hinged parts. The area provided with the hinged parts comprises at least one main hinged part and at least one sacrifice hinged part, and the sacrificehinged parts are shorter than the main hinged parts. When the air bag expands, the sacrifice hinged parts tend to generate the yield phenomenon to absorb impact force during air bag expanding. The yield strength of the sacrifice hinged parts is smaller than that of the main hinged parts, and the sacrifice hinged parts bear the impact force before the main hinged parts. Thus, it is ensured that the main hinged parts cannot be broken, the safety of the airtight device is improved, and the risk of main hinged part breakage is reduced.