In this paper, the authors proposed 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.