The sixth member of Yanchang Formation of Upper Triassic is the major exploration target zone in Baibao area,Ordos Basin,which is typical lithologic-stratigraphic reservoir.The data ofthe castslice,SEM ,electron probe and other tests combining with the analysis of core and sedimentary facies were applied to analyze the diagenesis of Chang 6 sandstone reservoirs.It showed that the destructive diageneses m ainly include compaction-pressure solution, cementation and packing action,and that the constructive diageneses mainly include rim agglutination of chlorite and dolomite at the early stage,denudation and cataclasis.A ccording to the association types and evolutionary sequences of the diagenesis pattern,it is concluded thatthe diagenetic evolution of Chang 6 sandstone has already been in stage B of the middle diagenetic stage.Based on the diagenetic evolution pattern,the diagenetic facies was studied,and it could be divided into three types, including compaction-cementation diagenetic facies association, compaction diagenetic facies association and cementation-denudation diagenetic facies association.The relationship between these diagenetic facies association and reservoir development，as well as the regional distribution were analyzed，which could provide reference for the reservoir-forming analysis and reservoir prediction of lithologic-stratigraphic reservoirs.