This research is mainly on the basis of eight backbone wells including Longtan1 well and Zhenjia1well.Through drill core observation,well logging interpretation, microscopic slice identification and sedimentary microfacies analysis, combined with regional tectonic information and reference to the predecessors’ achievements, the sedimentary facies of ordovician Majiagou Formation in the northern Shaanxi salt basin has been recognized and classified.Also compilation and revision of lithofacies paleogeographic maps in each majiagou period from 1 to 6 has been completed.The results show that carbonate platform facies accouts for the most in majiagou formation of shaanxi salt basin, along with open platform and restricted evaporate platform subfacies development. The restricted evaporate platform closely related to potash formation is further divided into microfacies,such as dolomitic flat, salt lake, gypsum lake, desalination lagoon.Paleogeographic research shows that 6 paleogeographic units including land, dolomitic flat, salt lake, gypsum lake, open platform and shallow lake has been developed during Majiagou Stage from 1 to 6.The distribution of paleogeographic unit in each member is different respectively. The key strata of potash forming in the research area, in another word Om56appears the paleogeographical pattern of “two depressions and one lifting” which is the core factor for the formation of secondary salt sag. Looking for favorable potash sag is the key to breakthrough in the northern Shaanxi salt basin.