A fixing device includes an encircle element having lateral sides and two flap portions extending outwardly from the lateral sides and formed with a through hole; a positioning element fixed securely to the encircle element and having lateral sides extending parallel to the lateral sides of the encircle element so as to define restriction chambers, wherein one lateral side of the positioning element has a longitudinal length greater than the lateral sides of the encircle element and is formed with an upward notch; and a locking plate having a first end connected pivotally to the through hole in the encircle element and a second end formed with a downward notch such that the locking plate is pivotable relative to the encircle element between an unlocked position, in which the notches disengage relative to each other and a locking position, in which the notches engages relative to each other.