The Jianghan Basin is a typical Paleogene salt lake basin in eastern China. Micro- crystalline dolostone in the Upper Eocene salt-bearing series in the Qianjiang sag, Jianghan Basin is composed of micro-crystalline dolomite, and the dolomite is red and orange-red in cathodoluminescence images. Associated minerals are primary glauberite, halite and terrigenous mud; while calcite is absent or rare. The X-ray diffraction analysis shows that the micro-crystalline dolostone has an average CaCO3 mole content of 57.30% and an average degree of order of 0.40, revealing the characteristics of higher calcium content and lower degree of order. It is closely intergrown with glauberite, indicating that the aqueous medium in the depositional environment of micro-crystalline dolostone was hypersaline water or dilute brine. The temperature of water in the depositional environment calculated according to the oxygen isotope data averaged 30.5℃ and the minimum homogenization temperature of inclusions in primary glauberite crystals associated with micro-crystalline dolomite was 44℃. The two groups of temperature data are close, indicating that the temperature of the water in the paleolake environment during the deposition of micro-crystalline dolostone ranged from 30.5℃ to 44℃. Thus it may be inferred that the micro-crystalline dolostone in the Upper Eocene salt-bearing interval in the Qianjiang sag is peneprimary dolostone formed directly by chemical precipitation in a high-temperature, high-salinity, high-Mg/Ca ratio and perennially meromictic salt lake.