The present invention discloses an automatic document feeding scanning device including an automatic document feeder, a flatbed scanner and a power switching mechanism. The automatic document feeder has a pickup arm and a feeding gear, the flatbed scanner has a scanning module and a scanning gear, and the power switching mechanism has two driving elements and a planet gear module. When the two driving elements rotate in the same rotating direction, the planet gear module is swung to a position near the scanning gear and engaged with the scanning gear such that the power switching mechanism transfers a driving power to the automatic document feeder. When the two driving elements rotate in different rotating directions, the planet gear module is swung to a position near the feeding gear and engaged with the feeding gear such that the power switching mechanism transfers a driving power to the flatbed scanner.