An automatic document feeding scanning device including an automatic document feeder, a flatbed scanner and a power switching mechanism is provided. The automatic document feeder has a pickup arm and a feeding gear, the flatbed scanner has a scanning module and a scanning gear. When the scanning module is moved and the movement of the scanning module is obstructed to enable the scanning module to take a first reacting force, the power switching mechanism is disengaged from the scanning gear in response to the first reacting force and transfers a driving power to the automatic document feeder. When the pickup arm is swung and the swing of the pickup arm is obstructed to enable the pickup arm to take a second reacting force, the power switching mechanism is disengaged from the feeding gear in response to the second reacting force and transfers the driving power to the flatbed scanner.