Abstract: A thick algal reef dolostone of Miocene was penetrated by the Well Xiyong-2. Routing work such as thin sections, scanning electron microscopy (sem), X-ray powder diffraction and Computed Tomography (CT) scan are conducted in combination with conventional reservoir property analysis for study of reservoir characteristics and diagenesis. The reservoir rocks are mainly composed of fine crystalline dolomite, and Intergranular dissolved pores, algae frame pores, moldic pores, intergranular pores and biological body cavity pores are observed as the main reservoir space. Porosity changes mainly within 8.39%~37.71%, while permeability changes within 0.17×10^-3~3291.9×10^-3μm², and both are quite high. In the well, the types of disgenesis such as compaction, cementation, dolomitization, recrystallization and dissolution are observed. Compaction and cementation are the main factors to reduce the porosity after deposition. Dolomitization, recrystallization and dissolution are the factors to improve the porosity by changing the textures of pores. Research shows that Miocene dolomite in the well Xiyong-2 was formed in the glacial stage by leaching of atmospheric fresh water.