Chang 6 oil reservoir set is the major exploration target in Fengdikeng-Hongde area of Ordos Basin. Because it has such petrological characteristics as fine grain size, low compositional maturity, poor psephicity and worse sorting, it is extremely important to carry out study on the diagenesis which controls the physical properties of reservoir. Combining with the data from plainlight microscope, scanning electronic microscope, cathodoluminescence, X-ray diffraction analysis and homogenization temperature test of fluid inclusion, the diagenesis of Chang 6 oil reservoir set was studied systematically. The result shows that the main diagenesis types include compaction, pressure solution, cementation, metasomasis and dissolution, and its diagenetic evolution lies mainly in the middle diagenetic stage A. Based on the diagenetic study and combined with clay mineral components, cement and growing situation of pores, the Chang 6 oil reservoir set is divided into five kinds of diagenetic facies, namely kaolinite cementation, chlorite cementation, ferrocalcite cementation, chlorite film plus feldspar dissolution and feldspar dissolution, among which the latter two kinds are the most favorable reservoir facies.