Abstract: The Ordovician strata constitute an important target of marine potassium ore in the east of Central paleo-uplift, Ordos Basin. Based on core, logging and seismic data, the authors conducted researches on the middle combination of Majiagou Formation (from 5th to 10th intervals of the fifth member of Majiagou Formation), in the aspects of sedimentary microfacies, evaporate distribution, formation mechanism and preservation condition of evaporate. The results indicate that the middle combination of Majiagou Formation was deposited in a restricted platform environment which can be further divided into four sedimentary microfacies, i.e., tidal flat, tide marginal beach, outer lagoon and inner lagoon. These microfacies are distributed around the central paleo-uplift orderly. The inner lagoon is the main depositional environment of the potassium evaporate. The sediments of the inner lagoon present obvious cyclicity. The sedimentary environment of the sub-member 6, 8, 10 were under the regressive evaporation condition and produced evaporite and dolomite, while the sub-member 5, 7, 9 were of transgressive setting and formed limestone. In the regressive period, the centripetal enrichment effect formed a large salt sag in Mizhi area, Shaanxi Province. Furthermore, under the mechanism of"salt deposited in deep water", a lot of thick potassium evaporate series were deposited. These potassium evaporate series were mainly distributed under the post-karst basin coincidentally. The analysis of the intensity and depth of the Caledonian karst indicates that karstification did little damage to these potassium evaporate series.