The present invention relates to an automatic document feeder having a mechanism for releasing paper jam by disconnecting the transmission device. The automatic document feeder includes a driving motor, a transmission device, a plurality of paper transfer rollers, a swing arm and a releasing arm. The driving motor is used for providing driving power. The transmission device and is coupled to the driving motor for transmitting the driving power. The swing arm is coupled to some of the transmission elements and operated in either a connection state or a disconnection state. The releasing arm is selectively contacted with the swing arm to have the swing arm enter the disconnection state or separated from the swing arm to have the swing arm enter the connection state. The paper jam is released when the swing arm is operated in the disconnection state.