The term "Black Ditch" (黑水溝) representatively characterizes the danger for Taiwanese Han ancestors crossing the Strait for immigration to Taiwan. Recently, however, some scholars misinterpret the term as the Penghu channel or a Kuroshio (黑潮)tributary . This article discusses in accordance with traditional literature and states that the Black Ditch refers to an ocean current located in the sea between Xiamen(廈門)and Pescadores Islands(澎湖). Further, it describes the relationship between the Black Ditch and the traditional marine writings of Heishuiyang(黑水洋), Luo-Ji(落漈), Ru-rio(入溜)and Ruo-Shui(弱水). This article also discusses the impacts and dangers posed by the Black Ditch for of navigating across the Taiwan Strait; finally, the research shall elaborate when the term Black Ditch appeared in the literature and how it evolved to symbolize the danger of crossing the Taiwan Strait.