Based on observation of hundreds of thin sections and SEM analysis, ten types of the reservoir space was found in Ordovician Majiagou Formation in the southern part of Ordos Basin, such as dissolved fracture, dissolved pores, intergranular pores, and intercrystalline pores, etc., in which intercrystalline dissolution pores and intercrystalline pores are the main types. During evolution of pores, dolomitization, corrosin and degypsification are favourable to formation of pores while cementation, compaction-pressolution, gypsification and dedolomitization are unfavourable and ruinous. Recrystallization is dualism. Analysis of environment, stages and characteristics of diagenesis has shown that pore evolution came through three stages: the formation and decrease of primary pores in early stage, the formation and filling of secondary pores in middle and late stages. Evolutionary process reveals that hypergene and embedding diagenesis stages are significant to form effective reservoir space.