This disclosure is related to a duplex scanning device. The automatic document feeder includes a paper feeder body, a paper inlet and a paper outlet. The paper cassette includes an upper paper tray and a lower paper tray adjacent to the paper inlet and the paper outlet. The cassette power module includes a motor set and a transmission part. The transmission part drives the paper cassette to move on one side of the automatic document feeder to control the lower paper tray to connect with the paper inlet or the paper outlet. Therefore, the double-sided scanning of the document is completed by two-pass duplex scanning.