An image acquisition module in a transmission device for a scanner is installed in a base of the scanner, a rotating wheel and a first roller are pivotally connected onto two inner sides of the base, and a driving roller is installed below the first roller. A conveyor is respectively put around the rotating wheel, the first roller and the driving roller, and a second roller is pivotally connected onto a place between the rotating and the driving roller and used for withstanding the conveyor between the rotating wheel and the driving roller to allow the conveyor between the rotating wheel and the second roller to be parallel to the conveyor between the rotating wheel and the first roller, and the conveyor is driven by the driving roller to drive the image acquisition module to move.