The South China Sea is the biggest marginal sea in the West Pacific. The environmental conditions are favorable there to the growth of reefs. The Dongsha uplift was a shallow water area far from provenance in the Miocene. The marine conditions including temperature, salinity and water depth were excellent for reef growth. Based on the integrated study of seismic, well logging and core data, and taking into account of relative sea level changes, the authors believe that there were plenty of reefs developed in different horizons in the Dongsha area, due to the complicated cooperation of relative sea level change and regional tectonic evolution, which made the reef rocks good reservoir with high porosity and permeability.