Reservoir sandstone of Chang 6, Chang 7 and Chang 8 formation of the Yanchang formation(Triassic) in Shancheng-Yanwu area of Ordos basin is typical of low porosity and low permeability, which has the characteristics of fine grain size, low composition maturity, poor roundness and middling sorting. Based on thin-section observation, X-ray diffraction, SEM, mercury injection and cathodolum inescence techniques, this article takes research on reservoir sandstone characteristics, diagenetic stage and diagenetic mode in Member 6 to 8 of Yanchang formation, analyses their influence on low permeability reservoir physical property. Meanwhile, the article indicates that mechanical compaction at the early diagenesis phase and ferroan calcite cementation at the late diagenesis phase are main factors making them very low permeable,  chlorite rim, grain dissolution and micro-fracture are believed to have improved the reservoir quality.