Abstract: Biogenetic Reefs are one of the most important oil and gas reservoirs in the South China Sea. Based on the characteristics of biological species, quantity, distribution and ecological features of main reef-building organisms revealed by the cores and slices of the Well XK-1, we studied in this paper the evolutionary history of reefs and related carbonate deposits of the Xisha Islands since Miocene. Data suggest that corals, calcareous algae and bryozoans are the three major reef-building organisms in the Islands since Miocene. According to their distribution pattern, combined with the established depositional model of the reef-beach complex, the environmental evolution of the islands is subdivided into 13 stages since Miocene by the authors. Data also suggests that reef-building organisms show a trend of evolution from corals to calcareous algae and bryozoans when the sea-level is rising, and vice versa. There are three major and two subordinate reef-building periods in the Xisha Islands since Miocene. The three major ones are the depositional period of the 1st Member of the Sanya Formation in early Miocene, the depositional period of the Huangliu Formation in Late Miocene, and the late stage of the Yinggehai Formation in Pliocene to the deposition period of the Ledong Formation in Quaternary, The two subordinate periods are the early stage of the 2nd Member of the Sanya Formation in early Miocene and the Middle stage of the Meishan Formation in Middle Miocene respectively.