Abstract: In the South China Sea, dolomite is not only a kind of oil and gas reservoir, but also the deposits closely related to the genesis of reefal complex. In this paper, mineralogical analysis is made for the 600～750 m interval of core section of the Meishan Formation in the Xisha Islands. The research is focused on a new horizon of middle to deep dolomite, 600～1000 m from the well of Xike 1, which may become oil and gas reservoir in the future. A large number of rhombic crystals are found by SEM. EDS analysis suggests that the rhombic crystals are dominated by Ca and Mg in elements. XRD analysis further confirms that they are dominated by dolomite; excluding the surficial clastic deposits which consists of calcite. EDS detected some Si from the samples, indicating the mixing of terrigenous materials. However, no Si has been found on the smooth surface of dolomite crystals by EDS. The content of SiO₂ increases with MgO in the dolomite. It is speculated that there is terrigenous material input during the formation of dolomite in the regression period. XRD results found that thick dolomite of medium order is developed in 620～646.5 m of the core, and the proportion of calcite begins to increase from 646 m.