Abstract: Post-Neogene reef dolostone occurs widely in the Xisha Islands and a great amount of ankerite has been found in the well of Xiyong 2. However, the research on ankerite is rare in literatures. In this paper, the ankerite-dolomites in the interval of 407～570 m of the well Xiyong 2 are analyzed with X-ray powder diffraction upon the basis of previous research data. It is found that the Miocene ankerite mainly occurs in the first Member of the Upper Miocene Huangliu Formation of the well Xiyong 2, while the ferroan dolomite dominates the second member. The porosity of dolomite shows a positive relationship with the amount of ankerite. The higher the porosity of dolomite rock, the higher the ankerite content. The crystal cell parameters of the Mesozoic dolomite and ankerite in this region are large, suggesting that they are rich in calcium and iron. The order degree of the dolomite-ankerite in well Xiyong 2 is greatly affected by Fe²⁺ content and the increase in Fe²⁺ will reduce its order degree. Therefore, it is not appropriate to suppose that the dolomite in the well Xiyong 2 of Xisha is simply formed by penecontemporaneous dolomitization. There was a great deal of volcanic activity in the Xisha Islands during Late Miocene, and the magmatic hydrothermal fluid may provide enough Fe²⁺ and heat for the formation of ankerite. In the late diagenetic period, the first member of Huangliu Formation with good porosity was greatly affected by volcanic activities. The heat and rich Fe²⁺ input promoted the transformation of dolomite to ankerite, and a large number of ankerite was then formed. However, in the second Member of Huangliu Formation with poor porosity there formed only ferroan dolomites.