Abstract: Abstract：The Middle Ordovician Majiagou Formation in Ordos basin was uplifted and eroded because of the Caledonian movement, forming favorable karst reservoirs. Therefore, the study of the sedimentary characteristics is of the key importance in oil-gas exploration. Based on the methods of single factor analysis (including analysis of stratigraphic thickness proportions of dolomite, limestone, gypsum and mudstone) in combination with detailed analysis and statistics of drilling core and logging data, the authors compiled and drew the lithofacies paleogeographic maps of Ma521 submembers. The results show that Ma521 was in a relative regression stage, and developed mainly gypsum-dolomitic flat, dolomitic flat, mud-dolomitic flat and mud-bearing dolomitic flat. According to lithofacies palaeogeographic maps in combination with stratigraphic thickness of Ma 521 , there may be a shallow depression in the middle of the study area, and hence the authors built the sedimentary model of Ma 521 in this area. On such a basis, the lithofacies palaeogeography of Ma521 can be summarized as “flats outside the lands, widespread dolomitic flats, depressions in flats, gypsum in depressions”. According to the lithofacies paleogeography and regional geology of this area, the formation of favorable gas reservoirs has been discussed preliminarily in this paper.