Abstract: An expansible bag comprises a bag body and two inner leaves. The bag body is made of airtight material and provides an inner chamber therein with an inlet communicating with the external atmosphere. The inner leaves are membranes attached to an inner wall of the bag body and provide two pressing stripes at a lateral side thereof respectively in a way of keeping a respective clearance path joining a part thereof to the bag body at both lateral sides of the clearance path respectively. The respective clearance path communicates with the inlet so that the bag body can be filled with fluid.