The dolomite of the Ordovician in Tazhong area are widely distributed，which has a large thickness and is one of the most important reservoirs in Tarim basin. According to the size of the structure of the dolomite crystal and the original sedimentary features，six kinds of dolomite types are found: mud - fine silty dolostone，algal dolomite，dolomite particles， coarse dolomite，medium - coarse crystalline dolomite，mottled dolomite. The mud - fine silty dolostone and algal dolostone preserve the original sedimentary. The study of geochemical characteristics of Ordovician dolomite shows: high carbon and oxygen isotopes，the index of salinity is higher than normal sea water，high Sr and Na，with relatively low Fe and Mg，the characteristics of REE are similar，they are interpretated of the production of the penecontemporaneous dolomitation. Crystalline dolostone and coarse dolostone particles with a crystal is relatively large，with a clear replacement of the structure (such as fog light side of heart structure and ring structure) features.By the geochemical characteristics of rare elements，the carbon and oxygen isotopes turn to the negative of offset and the index of salinity is low with the development of the diagenetic.The low of the Sr and Na，and the high of the Fe and the Mn，the ∑REE is changed very big， REE distribution patterns and Ce， Eu anomaly similar to be interpreted as a buried phase of the role of the product of dolomite，and may have been affecte by the tectonic hydrothermal liquid in the process of dolomite. Based on the study of the mechanism of the origin of the dolostone，the different dolomitation models are established to adapt to the actual case in the Tazhong area.