Abstract: Conventional section, cast section, X-ray analysis, cathode luminescence and SEM samples were used to study the diagenesis of the eighth member of Yanchang Formation (Chang8) in Maling area of the Ordos Basin, analyze its effects on reservoirs, divide diagenetic facies, and to forecast favorable diagenetic facies. The Chang8 reservoir in the study area has experienced various diagenetic effects such as compaction, cementation, dissolution and metasomatism. Compaction and ferruginous carbonate cementation in the late diagenetic phase reduced the sandstone porosity and the permeability of the reservoir, while dissolution produced a lot of secondary dissolved pores and improved permeablility. According to diagenetic functions and pore types, the sandstone reservoirs in the study area were divided into five diagenetic facies including strongly-dissolved residual inter-granular pores+dissolution pores, weakly-cemented residual inter-granular pores, clay mineral cementation, carbonate cementation, and strong compaction, among which the first two facies are favorable for reservoir development.