Abstract: Ferroan dolomite is found in well Xiyong 2 of Yongxing Island and well Xike 1 of Shidao Island in Xisha Islands. Through three methods including X-ray powder diffraction quantification, polarized light microscopy and scanning electron microscopy, distribution and petrographic characteristics of ferroan dolomite in the aboved two wells were discussed. The X-ray powder crystal diffraction analysis and comprehensive study show that eleven layers of ferroan dolomites are developed in the well Xike 1 of Shidao Island, and eight layers ferroan dolomites in the well Xiyong 2 of Yongxing Island. It was found by polarized light microscopy that the Miocene dolomite in the well Xiyong 2 of Yongxing Island is dominated by bioclastic dolomite, and the dolomite crystals are powder-fine grained; the content of ferroan dolomites is higher in some layers, and the crystal is mainly microcrystal-powder crystal dolomite. Through observing and analyzing the casting thin slices , it is found that a large amount of clastic dolomite is developed in well of Xike 1 and the crystals are mostly fine-grain crystal structure, self-formed to semi-self-shaped, and some samples are black ankerite oxide or cement. Under the scanning electron microscopy, samples from well Xike 1 in Shidao Island and well Xiyong 2 in Yongxing Island show that the dolomite crystals present rhombohedral polyhedral structures, with a high degree of dolomification.