Objectives:The crystalline dolostone is the most high quality reservoir in the Middle Jurassic Buqu Formation in the Southern Qiangtang Depression. This article aims to discusses the genesis mechanism of dolomitization and explains the good physical properties of crystalline dolomite by the study of rock structure and texture, element analysis, carbon and oxygen isotope characteristics, fluid inclusion homogenization temperature and calculated temperature from oxygen isotopes and further study of dolomitization fluid temperature, nature and source.Methods: Based on the field core observation and microscopic thin section identification, combined with the anslysis of element composition, carbon and oxygen isotopes, homogeneous temperature and salinity of inclusions, the fluid and temperature of dolomitization are studied.Results: The results show that the crystalline dolomite in the southern Qiangtang Depression are with carbon and oxygen isotope values of 2.86‰~4.27‰ and -3.27‰ ~ -8.17‰, repectively, of which the average value of oxygen isotope is significantly lower than that of the dolomite with original fabrics. The main peak value of the homogenization temperature of crystalline dolomite is between 90~140 ℃, and the calculated temperature of oxygen isotopes is in the narrow range of 50~90 ℃. The homogenization temperature tends to increase with the size of dolomite grains, which is consistent with the change trend of calculated temperature from oxygen isotope.Conclusion: Two kinds of dolomitized fluids with different temperature are developed in the Buqu Formation of the Southern Qiangtang Depression. Dolomitization processes occur under closed conditions, and thus dolomite grows with the increase of geothermal temperature and is not affected by foreign fluids. Dolomitization in the closed system is the most important diagenetic mechanism for the formation of high- quality reservoirs in the Southern Qiangtang Depression.