Lacustrine dolomite that develops in the Lower-1st Member of Oligocene Shahehie Formation in Qikou Depression of Bohaiwan Basin consists of grain dolostone and micrite dolostone and commonly occurs in thin interlayers with pelyte or interbeds with other carbonate rock alternately. Analysis result shows that the lacustrine dolostone has a lower order degree, which is similar with that of penecontemporary marine dolostone, but it is rich in calcium as the Mg/Ca ratio of it is deviated from the usual value, which indicates that the dolostone formed under a condition of rapid replacement. In addition, the analysis result of stable carbon isotope values and "Z" values reveal that it has high lacustrine saltness. Based on the dolomitization mechanism,an original model of Shahehie Lower 1st Member lacustrine dolostone is proposed. Contrasted with the origin mode of marine dolostone, the lacustrine dolostone is characterized of smaller distribution area and unthorough dolomitization so as to limited hydrocarbon reservoir capability.