Abstract: Theoretical and practical research has been conducted on diagenesis and porosity evolution of low permeability reservoir in southern Bozhong depression. Based on thin-section and reservoir properly data from five exploration wells, diagenesis study and quantitative analysis on pore evolution have been finished. It is shown that in southern Bozhong depression, E₂S₃ Fm is dominated by low permeable reservoir ,with high feldspar contents, deep buried depth and complicated pore structure, which are important factors for poor reservoir quality. Through reformation of compaction, cementation, dissolution and metasomatism, the reservoirs are now in the middle diagenesis periods of A2 and B sub-phase, during which reservoir pores were lost due to compaction, and the reservoir quality became poor. This study has provided a reliable basis for understanding the reasons of tight reservoirs in this region.