This article presents a study into the Ba Thuy Long belief as a cross-cultural symbol. Based on historical documents such as the stories of deities, imperial edicts, and on the arguments of other scholars, the author reckon that Thuy Long is a poly-cultural symbol, and it was created by using and mixing some other ancient symbols of some ethnics. The most ancient stratum is the Poriak belief (sea deity) of Cham people. In company with the Southward movement of Viet people, with the sea trade of Chinese residents, Poriak belief was progressively mixed with Long Vuong belief, Ha Ba belief, Ca Ong belief into a new belief called Ba Thuy Long. The result shows that Ba Thuy Long is a poly-ethnical symbol created in the process of cultural exchange.