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Medium-coarse-crystalline dolomite is a major variant of dolomite, and its structural heterogeneity inside and outside the crystals makes it an ideal vehicle for studies of dolomite genesis, fluid evolution and elemental migration. In this paper, the medium-coarse crystalline dolomite from the fifth section of the Majiagou Formation of the Lower Ordos Basin was systematically studied by petrographic, cathodoluminescence and related geochemical methods. The rock is developed in the context of mud powder crystal dolomite and granular dolomite, possessing significant ring band structure, fluorite and quartz intergrowth. Compared with other dolomites, it is characterized by Fe and Mn enrichment and low Na and Sr contents, as well as a negative δEu anomaly and a strong negative δ18 O offset. Combined with the evolutionary history of diagenesis, it is believed that the medium-coarse-crystal dolomite is the end product of a series of diagenetic effects such as early dolomitization-atmospheric freshwater leaching-secondary burial recrystallization-deep burial dissolution. Electron microprobe analysis reveals the structural and compositional heterogeneity inside and outside the crystals. The formation of mediumcoarse- crystalline dolomite is associated with an increase in Fe-Mn content, a rise in the Mn/Sr ratio, a decrease in Sr, and a first rise and then a fall in Na concentration. This study provides a valuable reference for further research on the genesis of the medium-coarse crystalline dolomite. [ABSTRACT FROM AUTHOR]