Densification modes of tight sandstone reservoir of the 8th Member of Lower Shihezi Formation,Permian,in Su-6 Area of the Sulige Gas Field, Ordos Basin, are quantitatively analysised by means of slice identification, scan electron microscope,electron probe micro-analysis and energy spectrum,homogenization temperatures and raman microprobe research of fluid inclusions. The results are as follows: (1) Densification process can be divided into the following stages, compaction during early diagenesis and the 1st stage siliceous cementationthe 2nd stage siliceous cementation and grain-coating chloritethe 1st stage dissolution(hydrocarbon charge)compaction during late diagenesis and the 3rd stage siliceous cementation (densification of reservoir) the 2nd stage dissolution and the 4th stage siliceous cementationcementation and replacement of chloritecementation and replacement of carbonate. (2) Densification modes of reservoir can be divided into three types: densification of medium-coarse grained litharenite by strong compaction; densification of medium-coarse grained lithic quartzarenite by strong siliceous cementation and medium to strong compaction; densification of fine grained (feldspathic) litharenite by strong compaction and medium to strong siliceous cementation. (3) The main reasons for densification of reservoir are compaction and siliceous cementation. And the 2nd stage dissolution under enclosed environment has less ascension of porosity, which is another important reason for densification. (4) The two types of medium-coarse grained sandstones are densification after hydrocarbon charge, which experience injection of organic acid and make dissolved pore to be the major storage space. Fine grained (feldspathic) litharenite is densification before hydrocarbon charge,which experience weak dissolution. (5) Difference of densification modes lead to the porosity,permeability ,correlation of permeability and porosity of medium-coarse grained sandstone are significantly higher than that of fine grained sandstone.