Based on the cores, thin sections and experimental data of dolomite in Majiagou Submember-55 of Ordovician in east-central Ordos Basin,it is shown that the value of carbon isotope of dolomite is the same as micritic limestone, ranging from -1.40℅ to 0.14℅, within the distribution of Ordovician sea water. The value of oxygen isotope of dolomite ranges from -7.51℅ to -5.98℅ which is heavier than that of micritic limestone, but lighter than that of Ordovician sea water. The REE distribution pattern of dolomite is the same as that of micritic limestone which is characteristic of LREE enrichment and HREE depletion. Combined with the ancient climate, sedimentary patterns and sea level changes, the results are obtained as follows: ①The dolomite is mainly formed by the seepage reflux dolomitization and partly by the evaporation pump dolomitization; ②Dolomite developed widely in the higher terrain such as Central Uplift-Jingxi Flat and Jingdong Uplift during the regressive period, which is controlled by the sea-level eustacy and palaeogeomorphology; ③The dolomite fabrics is related with its depositional settings. Micrite dolomite widely developed in Central Uplift-Jingxi Flat and Jingdong Uplift, grain dolomite and micrite dolomite with gypsum scattered in the Central Uplift-Jingxi Flat, and calcite micrite dolomite and dolomite micrite are mainly distributed in the central depression and edge zones.