The dominance diversity not only indicates genura and species of fossil taxa, but also shows that the number of every species distributed evenly or oddly. The dominance diversity has an important indicative significance to division of community and discovery of deposition environment. The concept of dominance diversity is discussed, and the entropy of information function is selected as a calculation method. The data of Ostracoda are analyzed and counted, and the dominance diversities are calculated in the first, the second and the third Member of the Shahejie Formation of Depression in the Dongying Sag Jiyang. Contour maps are drawn. In all 11 communities are divided according to the data of diversity and dominance species, including Limnocythere armata, Phacocypris huiminensis, Pseudocandona boxingsis, Camarocypris elliptica, Huabeinia unispinata, Camarocypris ovata, Huabeinia chinensis, Camarocypris ovata, Huabeinia chinensis, Cyprois mina, Huabeinia obscura. The data of paleoautoecology and dominance diversity play a very important role in the division of biofacies and environments. So the environments of different periods in the Dongying Sag are divided with the help of above communities.