Abstract: The study of carbonate platform in Xisha area is of significance not only to oil and gas exploration, but also to the development of carbonate sedimentology. Based on the high resolution 3D and regional 2D seismic data, through the study of geomorphology, regional sedimentary tectonics and carbonate sedimentology, seismic marks for definition of carbonate platforms are established accordingly. Carbonate platforms and their distribution in deep-water areas in the northwestern South China Sea are described and the genetic processes and main controlling factors analyzed in this paper. Seismic reflections terminated on the top and bottom of a carbonate platform. According to the tectonic evolutionary background (Ⅰ) and geographical location(Ⅱ), the carbonate platforms in the deepwater areas in the northwestern South China Sea may be divided into two categories, i.e. offshore carbonate platforms (middle Miocene) and isolated carbonate platforms (late Miocene). According to the platform edge(Ⅲ) and platform slope gradient(Ⅳ), they are further subdivided into the offshore platform around island shelf, the offshore platform on gentle slope, the offshore platform on steep slope, the isolated platform and so on. The carbonate platforms started growing in Miocene. With time they climbed up from west to east. Geomorphology and sea level changes are the two main controlling factors over the growth and demise of carbonate platforms. Geomorphologic relieves are the major controlling factors. For examples, the uplift on slope may control the scale, distribution and migration of the platforms, the basement convex may control the type, facies and migration velocity of platforms, half graben, as an "clean water" container, is favorable for carbonate platform developing; while the shape of platform edge may control the distribution, type and size of high-energy facies of carbonate platforms. With sea level rise, carbonate platform became smaller in area and moved from west to east. As the results, atolls became matured.