Abstract: In order to thoroughly investigate the diagenetic mechanism and environmental evolution of ultra-deep reservoir of the Cretaceous Bashijiqike formation in Keshen belt, by using the thin section observation, cathode luminescence analysis,scanning electron microscope, fluid inclusion microthermometry and laser raman spectum, energy spectrum analysis, the major and trace element analysis and other technical means, combined with diagenetic mineral, this paper portrays the characterization of diagenetic evolution of ultra-deep tight sandstone. The results show that ultra-deep reservoir had experienced alkaline-weak acidic-alkaline-acidic diagenetic environment. The combination of diagenetic fluid and diagenetic mineral: clay coating stained by iron and crystal carbonate in the early stage of alkaline fluid environment; dissolution of plagioclase and quartz overgrowth under the environment of epi-diagenetic; return to the alkaline environment with quartz dissolution, anhydrite cementation, dolomite cementation and feldspar overgrowth; in the late acidic environment with the dissolution of potassium feldspar, micro-crystalline quartz and illite. Alkaline cement products have occupied a large number of pores and the cracks formed in the early stage, which seriously damaged the permeability of the reservoir, the porosity was reduced by 10%~12%. While the acidic fluids carried by lately hydrocarbon charging can dissolute the acid-solube minerals and form dissolved pore and fracture network with the porosity increasing by 2%~3%. Therefore, the effective reservoir space of the ultra-deep reservoir is controlled by early alkaline and late acidic diagenesis environment. The alkaline diagenetic products provide the material basis for the lately acid corrosion, and the late acidic diagenetic environment plays an important role in the late improvement of the physical properties of the ultra-deep reservoir. The acid fluid is mainly controlled by atmospheric water infiltration in epi-diagenetic stage and lately hydrocarbon charging