A composite film for packaging foods enable to regulate pressure inside automatically when being heated and the progress of making it are provided. The composite film comprises a substrate film provided with a plurality of micro-gap regions processed by impressing process and an adhesive film attached to an airtight film. The composite film is applied to a food packaging devices which has an airtight structure. In use, the food packaging device can receive foods for microwave heating while the airtight film and the micro-gap region are used to prevent the food packaging device from breaking while preventing the steam vast losing and foods from becoming drier.