Based on the data from cores, thin-sections, porosity and permeability, and geochemistry, three sets of grain beach dolomite reservoirs were developed under the Ordovician gypsum salt in east-central Ordos Basin. They were the Lower Ordovician Majiagou Member-2, Member-4, and Submember-57+9. According to the lithofacies palaeogeography, the grain beach usually distributed around the depression (Member-2) and the secondary depression (Submember-57+9) in the platform, especially on the relative uplift belts. The types of dolomite reservoirs are mainly intergranular pores, intercrystalline pores, intercrystalline dissolution pores, dissolution pores and caves, and fractures. 3-D computed tomography shows that the dolomite reservoir has excellent porosity and permeability. The genesis of the grain beach dolomite is mainly the osmotic reflux of evaporative seawater in shallow burial diagenetic environment. The development of the dolomite reservoir is mainly controlled by three factors:the distribution of the grain beach under the sequence framework, the karstification, and the fracture system.