The paper discusses the discourse and geographical imagination of ”hou-shan”, or the back of the mountain, in the Qing documents. It first describes the geographical distribution of the place called ”hou-shan”, and then focuses on the changing geographies of ”hou-shan” as the eastern half of Taiwan. The paper argues that the official discourse presents ”hou-shan” as an uncivilized space beyond imperial administration, indicating a cultural hierarchical order in the pre-modem society. The Gazettes usually describes ”hou-shan” as a place where we expect to observe unbelievable or mysterious phenomena. In other words, ”hou-shan” as unknown geography greatly satisfies the folk imagination toward strangeness.