A fixing device includes a casing defining a receiving space and having an open end, and a movable element having a protruding end protruding from the open end. The movable element is movable relative to the casing along a first direction to vary a protruding distance of the protruding end from the open end of the casing. A plurality of first positioning portions are provided on one of the casing and the movable element. The second positioning portion engages interferentially and releasably a selected one of the first positioning portions along a second direction that is perpendicular to the first direction to restrict movement of the movable element relative to the casing.