A thick document conveying device includes a paper pickup mechanism. The paper pickup mechanism has a pick roller base, a pick roller axle, a pick roller, a switching assembly, a first universal coupling, a driving mechanism, a second universal coupling, a bearing and a gear. The pick roller axle and the pick roller are located in the pick roller base. The switching assembly is located under the pick roller base. The first universal coupling is connected to the pick roller axle. The driving mechanism is sleeved on the first universal coupling. The second universal coupling is connected to the driving mechanism. The bearing is connected to the driving mechanism. The gear is connected to the bearing. As described above, both a thick document and a thin document can be adapted.