Chang7 reservoir in Dingbian Oilfield of Ordos Basin has abundant oil reserves，but its tightness and anisotropy restrict the effective exploration and development. Based on thin section，scanning electron microscope（SEM）analysis，the rock compositions，pore types，physical properties，and diagenesis of Chang7 tight reservoir are studied. The porosity evolution model of tight sandstone reservoir are established at Chang7 by using the diagenetic evolution events and geological comprehensive response，to clarify the evolution of diagenesis and porosity. Meanwhile，influences of diagenesis at different depths on the porosity are compared，which provides a theoretical basis to explore the relatively dominant reservoirs in the study area. The results show that：The clastic rocks of Chang7 tight reservoirs in the study area are mainly feldspar sandstone and feldspar lithic sandstone，mainly containing clay minerals，carbonate，and other cement. The effective pore types are mainly secondary dissolved pores and residual intergranular pores，indicating a typical tight sandstone reservoir with ultra-low porosity and ultra-low permeability. The diagenetic evolution of Chang7 reservoir in the study area has gone through stages of rapid compaction-early cementation，cementation-early dissolution，mainly dissolution-pore increase，compaction-late cementation，etc. This suggests that Chang7 oil reservoir in the research area is currently in the middle diagenetic stage A. Compaction is the main controlling factor leading to pore decrease and tight reservoir formation，with an average pore reduction of 20.4%. The average pore reduction by cementation is 14.02%，and the cementation in the late stage is the key to form tight reservoirs. The dissolution increases the porosity by 5.87% on average，which delays the densification. The sandstone compaction and cementation in the bottom of Chang71 and middle of Chang72 are relatively weak，and the dissolution increasing pore volume is relatively strong，which indicates a preferred reservoir for exploration and development in this area.