The Chang8 tight reservoir of Triassic Yanchang Formation in Fuxian area of the southern Ordos Basin was taken as an example to systematically study the reservoir petrology characteristics，reservoir physical characteristics and diagenesis，and to discusses the influence of diagenetic facies types and differential diagenesis on reservoir physical property by thin section observation，scanning electron microscope（SEM）and X-ray diffraction. The results show that the main rock lithology of Chang8 tight reservoir in the study area is ultrafine to fine arkose with complex and different diagenesis. Diagenetic facies of the reservoir can be divided into three types：mechanical compaction diagenetic facies，carbonate cementation diagenetic facies，and weak compaction-weak cementation diagenetic facies. The sand of mechanical compaction diagenetic facies becomes tight reservoir mainly due to the mechanical compaction in the early stage of diagenesis and are mainly distributed in sand bodies with weak hydrodynamics. The sand of carbonate cementation diagenetic facies are densified in the early stage of diagenesis due to the massive precipitation of carbonate cement. The sand of weak compactionweak cementation diagenetic facies are mostly distributed in the sand body with strong hydrodynamics in the middle or lower part of the distributary channel. It has a high content of rigid grains and forms a large amount of chlorite cement in the early stage of diagenesis. Therefore，the ability to resist compaction is significantly improved，and the chlorite membrane inhibits the formation of other types of cement to a certain extent，thereby providing sufficient space for pore fluid activity，resulting in the development of dissolution，which constitutes a relatively high-quality reservoir in the study area.