The present invention is a self-acting valve, and it may specifically be used for air mattresses. This self-acting valve consists of a housing, a membrane and a base. After a quick inflation, the elastic airtight membrane positioned within the housing reverts to its original shape and instantly shuts off the outlets, which prevents air from leaking out. This valve is able to operate automatically to form a perfect airtight seal. The structure of this valve is simple, reliable, and practical, which makes it a new type of self-acting valve for an air mattress.