Based on data of conventional identification of several hundreds thin-sections from 16 wells in the southern part of Ordos Basin, the types and characteristics of the diagenesis of Ordovician Majiagou Formation reservoir are analyzed in combination with the depositional history and regional geological characteristics of the research region. It is conclude that the Majiagou reservoir rocks predominantly consist of limestone and dolostone. The limestone commonly are micritic limestone, microlitic and sparry grainstone and dolomite-stained limestone. The dolostone mainly are dolomicrite. Dolomitization, dissolution are the main constructive ones while recrystallization, compaction, pressolution, dedolomitisation and gypsification are the main destructive diagenesis in Majiagou reservoir. In addition, the evolutionary sequence of diagenesis in this region is discussed.