A transmission device of automatic document feeder includes a transmission gear assembly, a pickup transmission gear assembly, a paper-feed transmission gear assembly, a correction transmission gear assembly, a pressure transmission gear assembly and a turn-over transmission gear assembly. The transmission gear assembly includes a transmission roller, a first drive gear and a second drive gear. The pickup transmission gear assembly includes a pickup drive gear, a pickup swing arm and a pickup auxiliary swing arm. The paper-feed transmission gear assembly driven by the transmission gear assembly includes a paper-feed reverse gear and a paper-feed swing arm. And the correction transmission gear assembly and the paper-feed transmission gear assembly rotate towards the same direction. The pressure transmission gear assembly includes a pressure swing arm and a pressure actuating gear. The turn-over transmission gear assembly is driven by the first drive gear of the transmission gear assembly.