An automatic document feeder includes a paper transmitting path for guiding documents, a driving motor for providing driving torque, a pickup device disposed at the upstream of the paper transmitting path, an output device disposed at the downstream of the paper transmitting path and transmitting rollers disposed along the paper transmitting path for transmitting the documents, a first driving gear disposed between the pickup device and the output device, a transmitting gear swing arm equipped with a transmitting gear set and axially hinged with the first driving gear to swing according to the driving torque between a first position meshing the transmitting gear set with the pickup device and a second position idling the transmitting gear set, an actuator connected with the output device and a braking device disposed on the output device for locking the transmitting gear swing arm according to the position of the output device.