Based on thin section analysis, scanning electron microscopy analysis, cathodoluminescence analysis and physical property analysis, the authors studied the sandstone composition, the relative importance of chemical diagenesis and physical diagenesis, authigenic minerals, pores and physical properties to reveal the formation mechanism of Chang 6 sandstone reservoir with low-permeability in the upper Triassic Yanchang Formation of Huaqing area, Ordos Basin. The authors selected 15 diagenetic parameters to quantitatively evaluate and divide the diagenetic facies of Chang 6 sandstones on the basis of an integrated study of sedimentary facies and diagenetic processes. The results show that the Chang 6 sandstones have experienced strong physical diagenesis and weak chemical diagenesis, and hence the primary pores are the most important reservoir space; in addition, it is the compaction and calcite cementation that cause the porosity reduction and poor reservoir quality. The reservoir quality of sandstones affected by the northeastern provenance is significantly better than that affected by the southwestern provenance. Furthermore, the southwestern sandstones, showing weak cementation and strong compaction diagenetic facies and strong calcite cementation and moderate-strong diagenetic facies, have poor reservoir quality, whereas the northeastern sandstones, especially the middle sand bodies, which show chlorite cementation and weak-moderate compaction diagenetic facies, have good reservoir quality and are thus recommended as preferred exploration zones.