This article is the first part of a serial research on learning about the issues related to the Southeast Asian Sea represented by various maps which were drawn during successive historical periods of China, from the utmost antiquity to the period of the Republic of China. In the beginning, the author gives the brief review of the formation and development of Chinese ancient cartography from the beginning until mid-10th century. Then, the presentation of entire China's geographical and administrative maps during the two' dynasties of Song-Yuan, including large-sized maps carved in stone, sets of maps (Atlas), appendix maps in history books, and geography books... Most of those maps were made by high-ranking mandarins under the command of Chinese emperors. The consistent point which can be clearly seen in those maps is that they are all defined the southernmost border of China is Quynh Chau island (present Hainan island). This is conformable to the acknowledgement of the southernmost border recorded in contemporary Chinese official history and geography books.