The invention discloses an E-commerce express packing box with a size-adjustable built-in space. The box body of the express packing box is manufactured by folding a box body component. The box body component comprises an end plate I, a side plate I, an end plate II and a side plate II, which are arranged linearly and connected in sequence. Line pressing marks are arranged between the end plate I and the side plate I, between the side plate I and the end plate II and between the end plate II and the side plate II. Inner flaps are respectively arranged on two side edges of the end plate I and two sides of of the end plate II. Each inner flap comprises two side folding strips parallel to each other and a middle folding strip arranged between the two side folding strips. Each middle folding strip and each side folding strip both comprise two parallel line pressing marks. U-shaped positioning plates, which are used for fixing the folded middle folding strips and the folded side folding strips, are arranged at positions, close to connecting ends, of the middle folding strip and the side folding strips. The E-commerce express packing box is convenient to manufacture and process, pollution-free, easy to recycle and low in packing cost, facilitates warehouse logistics, and has the advantages of integrally buffering and fixing a product and adjusting the size of the built-in space.