Abstract: Carbonate buildups started growing up in the Late Oligocene on the edge of the Xisha Islands and structural highs. Sequence analysis and drilling well facies analysis suggest that a carbonate buildup may consist of various facies, such as the lime sand island facies, reef flat facies, bindstone facies, reef framework facies and backreef lagoon facies etc. In seismic profiles, the carbonate platform usually occurs as a positive landform with high amplitude at the top, low amplitude and chaotic reflection inside, and an ambiguous boundary at the bottom. Large scale carbonate buildups usually have high amplitude, continuous reflection and a flat top. We proposed in this study a sedimentary model for the Xisha carbonate buildup with respect to relative sea level changes. The rise of relative sea level leads to the retreat of carbonate buildup toward the structure highs, while dropping of the relative sea level causes the lateral accretion of the carbonate buildup towards the sea, leaving a lagoon behind the buildup. Besides, tectonic movement, terrigenous clastic input and temperature and salinity of sea water also influence the development of the carbonate buildup.