Abstract: The contents, distribution pattern and elemental anomalies of rare earth elements in carbonates are the records of surrounding water. Corals are characterized by high resolution and high stability of rare earth elements and may faithfully record the geochemical characteristics of the surrounding seawater. In this paper, we analyzed the coral reefs from 142 to 84 ka collected from the Yongxing Island of the Xuande Atoll of Xisha Islands, South China Sea. Trace element contents, especially the rare earth element contents and their distribution patterns are used in this paper to determine the characteristics of the sea water, in which the coral reefs grew. Results show that, since 142 ka, most of the coral reefs in the Yongxing Island has a normal rare earth element distribution pattern of marine carbonates, characterized by LREE depletion, negative Ce anomalies and high Y/Ho ratios, indicating an environment of open shallow sea. In contrast, the coral skeletons in depth of 23 m 114 ka have similar LREE depletion, negative Ce anomalies and high Y/Ho ratios, but positive Eu anomalies. This suggests that certain amount of hydrothermal fluid has been input during the growth of corals. Based on the model calculations, it is inferred that at least 0.1% of hydrothermal fluid has been added to the open seawater during that time. The hydrothermal fluids may be related to the volcanic activities observed at Gaojianshi island or Hainan island.