To make clear the physical and chemical changes caused by CO₂ flooding and geological storage ，the mineral composition of reservoir and the law of interaction between CO₂，rock and formation water in Yanchang oilfield was studied by using SEM，XRD，EDS ，ICP-AES and IG. The results showed that the reservoir rock of Yanchang oilfield was composed of 18.7% quartz，18.2% potassium feldspar，30.1% plagioclase，0.5% calcite ，4.4% laumontite and 28.1% clay minerals. The average salinity of formation water was 71.34g/L with CaCl₂ type ，and the pH value was 5.5. After the reaction of CO₂，rock and formation water，the corrosion of rock surface became worse with increasing partial pressure of CO₂. At reservoir temperature of 44℃，when the reaction pressure increased from 0 to 20MPa，the content of C element in rock surface fluctuating rose，while that of Na，K elements gradually decreased，the salinity of formation water increased from 95210 mg/L to 107063 mg/L，and the mass concentration of Na⁺、K⁺、Ca²⁺、HCO₃^-showed an increasing trend. When the reaction pressure increased from 5 to 20MPa，the corrosion rate of piece rock increased first and then decreased. When the pressure was 10MPa，the maximum corrosion rate was 0.72%. The higher pressure，the higher solubility of CO₂ in water and corrosion rate was. The higher temperature and salinity ，the lower solubility of CO₂ in water was. With increasing reaction pressure ，the dissolution of potassium feldspar and plagioclase in sandstone by CO₂ gradually increased.