An adjustable working area cutting machine includes a worktable, a cutting unit, a movable guide rail unit, a fence unit, and a locking unit installed on one of the worktable and the movable guide rail unit. The worktable includes a first rail. The movable guide rail unit includes a channel bar coupled to the first rail, and a second rail. The locking unit is fixed to the other of the worktable and the movable guide rail unit. The fence unit includes a fence arranged to be located on the top side of the worktable, and a slide coupled to the second rail. By moving the movable guide rail unit relative to the worktable, the working area of the cutting machine is relatively extended. To cut a large size workpiece, the position of the movable guide rail unit is adjusted relative to the worktable to extend the working area.