Majiagou Formation is an important Ordovician gas-bearing formation in Ordos Basin. The diagenesis types and the petrogenetic conditions of carbonate rocks of Majiagou Formation in southern Jingbian tableland are studied. The result shows that dolomitisation, ancient karstification and cemented metasomatism are dominated factors for the reservoir. According to various diagenesis characteristics and paragenetic sequence relations, it is concluded that this set of carbonate rocks mainly experienced four diagenetic stages: penecontemporaneous diagenetic stage, epidiagenetic stage, early diagenetic stage and anadiagenetic stage, and at present, it reached the later period of anadiagenetic stage. Based on the research of the diagenesis and combinatory sequence, the diagenetic facies is studied, and it is roughly divided into three diagenetic facies, including hypergene ancient karst-early filling cementation diagenetic facies, hypergene ancient karst-buried ancient karst diagenetic facies and buried cementation diagenetic facies. Hypergene ancient karst-buried ancient karst diagenetic facies has the best reservoir quality. Based on the studies on the influencing factors, the plane distribution of the diagenetic facies is definited, which provides foundations for reservoir evaluation and prediction.