Airtight sheath being adjustable in size for packing

An airtight sheath being adjustable in size for packing and made of at least two outer films heat-sealed together, the airtight sheath comprising:

• a first buffering unit comprising a plurality of first air columns;
• a second buffering unit comprising a plurality of second air columns arranged apart from the first air columns, and two terminating sides formed at opposite sides of the second buffering unit, the terminating sides being heat-sealed to two opposite sides of the first buffering body;
• a bottom buffering unit comprising a plurality of bottom air columns, two opposite sides of the bottom buffering unit integrally respectively connecting the first and second buffering units such that the bottom air columns correspondingly communicating with the first air columns and the second air columns to enable air to flow thereamong, the first, second, and bottom air columns cooperatively surrounding to form a packing space therein;
• and
• a foldable unit integrally extending from one side of the second buffering unit opposite to the bottom buffering unit, and comprising a bending body, and a plurality of abutting air columns extending from and communicating with the bending body, the bending body being bendable from the second buffering unit so that the foldable unit is foldable into the packing space, where the packing space is capable of being narrowed by the abutting air columns and the first buffering unit;
• wherein the foldable unit further comprises a plurality of foot air columns connected to ends of the abutting air columns opposite to the bending body, and the foot air columns are being propped against the first buffering unit and bend toward the abutting air columns when the foldable unit is folded into the packing space.