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; the first tube having a non-circular cross section, a first connection end formed to the first end of the first tube, the second tube having a non-circular cross section that is matched with the non-circular cross section of the first tube, a second connection end formed to the first end of the second tube, multiple holes defined through a wall of the second tube; the control unit including an inner part, a switch and a sleeve, the inner part being an oval-shaped tube and having a C-shaped cross section, the inner part including a passage defined axially therethrough, the first connection end of the first tube inserted into the passage from a first end of the inner part, the second connection end of the second tube inserted into the passage from a second end of the inner part and slidably inserted into the first connection end, a slot defined axially through a wall of the inner part and communicating with the first end and the second end of the inner part to form the C-shaped cross section, a support portion formed between two inside walls of the slot; the switch is pivotably connected to the inner part, the switch including a first end and a second end, a first contact portion formed to a top face of the first end of the switch, a rod connected to an underside of the second end of the switch, the rod removably inserted into one of the holes of the second tube, a portion of the underside of the second end of the switch being supported on the support portion when the rod is removably inserted into one of the holes of the second tube, and the sleeve including a tunnel defined axially therethrough, the inner part inserted into the tunnel of the sleeve, a tongue formed to a wall of the sleeve and located corresponding to the slot, the tongue including a second contact portion formed to an inside thereof, the second contact portion removably engaged with the first contact portion, when the tongue is pressed toward the slot, the switch is pivoted to remove the rod from the hole 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; the first tube having a non-circular cross section, a first connection end formed to the first end of the first tube, the second tube having a non-circular cross section that is matched with the non-circular cross section of the first tube, a second connection end formed to the first end of the second tube, multiple holes defined through a wall of the second tube; the control unit including an inner part, a switch and a sleeve, the inner part being an oval-shaped tube and having a C-shaped cross section, the inner part including a passage defined axially therethrough, the first connection end of the first tube inserted into the passage from a first end of the inner part, the second connection end of the second tube inserted into the passage from a second end of the inner part and slidably inserted into the first connection end, a slot defined axially through a wall of the inner part and communicating with the first end and the second end of the inner part to form the C-shaped cross section, a support portion formed between two inside walls of the slot; the switch is pivotably connected to the inner part, the switch including a first end and a second end, a first contact portion formed to a top face of the first end of the switch, a rod connected to an underside of the second end of the switch, the rod removably inserted into one of the holes of the second tube, a portion of the underside of the second end of the switch being supported on the support portion when the rod is removably inserted into one of the holes of the second tube, and the sleeve including a tunnel defined axially therethrough, the inner part inserted into the tunnel of the sleeve, a tongue formed to a wall of the sleeve and located corresponding to the slot, the tongue including a second contact portion formed to an inside thereof, the second contact portion removably engaged with the first contact portion, when the tongue is pressed toward the slot, the switch is pivoted to remove the rod from the hole so that the second tube is movable relative to the first tube.