A multi-functional retaining strap has two strap surfaces. Each strap surface is installed with attachable tapes. The multi-functional retaining strap comprises a short extending retaining strap having one end fixed to a middle section of the strap surface of the multi-functional retaining strap and another end which is not fixed; a surface of the short extending retaining strap being an attachable tape; an inner folded strap attached to one of the strap surfaces; at least one surface of the inner folded strap being an attachable tape; and an outer folded strap having two ends the inner folded strap; a gap being formed between the inner folded strap and the outer folded strap; at least one surface of the outer folded strap is an attachable tape; an auxiliary strap for prolonging the length of the short extending retaining strap.