The Majiagou Formation in Fuxian area, Ordos Basin mainly deposited evaporation and tidal flat carbonate consisting of dolomites and gypsum-dolomite. The core observation, conventional thin section and casting thin section, X diffraction, carbon and oxygen isotope analysis combining with reservoir properties are used to study the carbonate diagenesis types and diagenetic environment of the fifth member ofMajiagou Formation in Fuxian area. The result shows that the carbonate rocks of the fifth member ofMajiagou Formation experienced the diagenetic environment of sea water, mixed water and fresh water. Dissolution and reflux dolomitization are constructive to the reservoir pore development, while mineral filling, dedolomitization and compaction-pressure solution are destructive.