A great thickness of evaporite strata were developed in the Ordovician in eastern Ordos BasinQuite a few researchers have proposed different views about their genesisThis paper presents another point of view in which the evaporites were deposited under the condition of the saline basin's periodical isolation from the open seaThis hypothesis is based on distribution patterns of the microfacies，profile architecture and geochemistry characters of the evaporitesThe idea of “reinfluxing and redissolving",is developed for explaining the origin of the thin black claylayers in thick saline rocks and the origin of various colors of the evaporitesThe lack of sylvitecarnallite layers is also consideredA new concept，“dissloving deposition bed"，is proposed to explain a special mud layer in evaporite strataThe microfacies of the Ordovician in this area show that some biobuildups including reefs or biomounds,grew on the elevated areas underwater along the eastern edge of the basinDuring the period of sea level falling，it acted as a barrier between the halite depression and the open sea that kept the halite depression isolated from the open sea，and brought it into the stage of evaporation drying to form halite bedsThe abrupt event of seawater reinfluxing，as a result of sealevel rising，could make part of the formerly formed halite bed dissolving(reinfluxing and redissolving) At the same time,the undissolved residues,dispersed in halites，gathered to form a bed(dissolving deposition bed),which may prevent the underlying halite from further dissolvationThe process of reinfluxing and redissolving may be the main reason for the lack of evidence of typical evaporation dryingThe sequence distribution style of carbonate and evaporites，weremainly controlled by regional sealevel change cycleCarbonates were formed during transgressive period or highstand period,while evaporites were formed during lowstand period