A cable tightening device includes a fixed unit, a rotation member rotatably mounted on the fixed unit, a movable unit mounted on the rotation member and movable relative to the fixed unit to drive the rotation member to rotate relative to the fixed unit in a oneway direction, and a locking unit mounted between the fixed unit and the movable unit to releasably lock the movable unit onto the fixed unit. Thus, the movable unit is locked onto the fixed unit by the locking unit to lock the cable tightening device and the cable so as to protect the cargo wound by the cable, thereby achieving an anti-theft purpose.