Guo Shou-jing, the famous scientist in the Yuan Dynasty, took charge of a large-scale latitude surveying in 1279 A.D. which is well-known as Si Hai Ce Yan(Surveying at four-sea areas) in the history of Chinese geodetic astronomical surveying. 27 observation stations were established at that time over a vast range of 11.000 Li long from north to south and 6,000 Li wide frow east to west. Nanhai(the South China Sea) station played a very important role in surveying since it was the southernmost amony 27 stations and held by Guo himself. This srticle discusses the problem of latitude surveying at Nanhai Sea from the aspects of historical geography and geodetic astronomical surveying. It consists of three perts. In the first part, the author expounded from the view of the geographical concept of the ancient name Nanhai that the surveying was carried out at the sea of Nanhai instead of in Guang Zhou which was also called Nanhai in the past. Secondly, the observation station at Nanhai was situated on the Xi Sha islands. Then the possibility of surveying there was further proved by the facts that the Chinese people exploited and managed the Xi Sha Islands before the Yuan Dyaasty. Thirdly, it discussed the instruments which were used in astronomical surveying by Guo and the reason of the difference of latitude values obtained at present and at that time on Nanhai. In the end, the author pointed out the historical fact that the Yuan government sent Guo himself to survey the latitude on the Xi Sha Islands is an undisputed evidence which shows that the islands in the south China Sea era the sacred territory of China since ancient times.