A document guiding mechanism includes a document supporting tray, a gearwheel coupled to the document supporting tray, a right guiding unit, a left guiding unit and a single-way latch means. The right guiding unit includes a right guiding wall positioned on the document supporting tray and a right linear gear extended from the right guiding wall and engaged with the gearwheel. The left guiding unit includes a left guiding wall positioned on the document supporting tray and opposite to and parallel to the right guiding wall, and a left linear gear extended from the left guiding wall and engaged with the gearwheel. The single-way latch means interconnects the document supporting tray and at least one of the right guiding unit and the left guiding unit. The right guiding wall and the left guiding wall can be kept at a suitable position by the single-way latch means.