An automatic document feeder includes an upper cover, a document pick-up module, and a conveying channel. The upper cover has a first hooking element. The document pick-up module is arranged between the upper cover and the conveying channel. In addition, the document pick-up module has a second hooking element. When the automatic document feeder is operated in a standby mode, the document pick-up module is swung to a position near the upper cover and the second hooking element is engaged with the first hooking element, so that the document pick-up module is fixed on the upper cover. Since the document pick-up module is not contacted with the document when the automatic document feeder is operated in the standby mode, the possibility of causing an erroneous action of the document pick-up module will be eliminated.