A retractable bicycle cross bar adaptor, comprising: a bar having a first tube, a second tube and a control unit, the first tube having a first end and a second end, the second tube having a first end and a second end, the control unit connected between the first tube and the second tube, a first hook connected to the second end of the first tube, a second hook connected to the second end of the second tube; a first connection end formed to the first end of the first tube, a second connection end formed to the first end of the second tube, the second connection end slidably inserted into the first connection end, and characterized by the control unit including a first part and a second part, the first part being a tubular part and having a head formed to a first end of the first part, outer threads formed to an outer surface of the first part, multiple plates extending from a second end of the first parts, the first part including a passage defined axially therethrough, multiple slots formed alternatively between the multiple plates, the first connection end inserted into the first part from the head of the first part and secured to the head of the first part, the second connection end inserted into the first part from the second end of the first part and movably inserted into the first connection end, each of the plates having an tapered outer face along an axial direction of the plate, the second part rotatably and movably mounted to the first part, the second part pressing the plates to clamp the second tube, or the second part releasing the plates so that the second tube is movable relative to the first tube. a bar having a first tube, a second tube and a control unit, the first tube having a first end and a second end, the second tube having a first end and a second end, the control unit connected between the first tube and the second tube, a first hook connected to the second end of the first tube, a second hook connected to the second end of the second tube; a first connection end formed to the first end of the first tube, a second connection end formed to the first end of the second tube, the second connection end slidably inserted into the first connection end, and characterized by the control unit including a first part and a second part, the first part being a tubular part and having a head formed to a first end of the first part, outer threads formed to an outer surface of the first part, multiple plates extending from a second end of the first parts, the first part including a passage defined axially therethrough, multiple slots formed alternatively between the multiple plates, the first connection end inserted into the first part from the head of the first part and secured to the head of the first part, the second connection end inserted into the first part from the second end of the first part and movably inserted into the first connection end, each of the plates having an tapered outer face along an axial direction of the plate, the second part rotatably and movably mounted to the first part, the second part pressing the plates to clamp the second tube, or the second part releasing the plates so that the second tube is movable relative to the first tube.