Abstract: Based on large quantities of field geological investigation, core observation and laboratory analysis, the authors studied in detail the diagenesis and diagenetic facies of Chang 101 reservoir in northern Shaanxi. The results show that Chang 101 reservoir of the study area is at stage A of middle diagenesis phase. Compaction and cementation by carbonate, clay minerals (kaolinite, illite), laumonite and silicon are the main factors for the loss of reservoir primary porosity. Chlorite cementation can prevent the pore from being filled. Dissolution of feldspar and laumonite minerals improves connectivity between pores. Physical properties of the reservoir are improved by these factors. There are eight types of diagenetic facies in this area. High quality reservoir is closely related to the diagenetic facies. Compaction facies, carbonate cementation facies, clay mineral cementation facies, quartz overgrowth facies and laumonite filling facies are unfavorable for the development of high quality reservoir. On the contrary, feldspar dissolution facies, chlorite cementation facies and laumonite dissolution facies play an important role in improving the physical properties of the reservoir.