Dolostones in palaeostrata suffered complex diagenesis, so the origin of dolomization has been a multiple solution problem. Core samples of the shallow-buried Neogene dolostones from drilled wells in the Xisha Islands, South China Sea, is chosen for studying the origin of dolomization to presume upon some revelation for the dolomization in palaeostrata. It is shown that the dolostones develop under the weathering crust, which means they were in the cycles of sea level decline during deposition, and it is proved that dolomization is controlled by periodic global eustasy. The fact that the beds with dolomitization correspond to relatively low palaeotemperature and relatively high palaeosalinity of seawater reveals that glacial periods lead to sea level declines and lead to seawater condensation, which brings about the dolomization. The alternating glacial/interglacial periods bring in sea level changes which lead to periodical changes of seawater nature and accordingly periodical dolomization to take place. That is the reason why there are the situations of periodical dolostone/ limestone superposition in the Xisha Islands. By correlating the origin of dolostones in Neogene strata with that of dolostones in palaeostrata, it is enlightened that the so-called burial dolostone distributed extensively in palaeostrata should be derive from penecontemporaneous dolostone that has suffered burial fluid reformation due to seawater condensation so that they exhibit the characteristics of burial dolostone. It is discovered also that the limestones above weathering crusts are of almost same pore type and porosity with the dolostones under weathering crusts in the Xisha Islands.