The origin of CO₂ and the alteration of CO₂ into clay minerals in eastern Shijiutuo symon fault of Bozhong depression were studied in this paper. The elevated δ¹³C and R/Ra values show the mantle-derivation of CO₂. The basin-controlling deep fault was the main migration pathway of CO₂ fluid. CO₂ fluid injection had obvious impact on clay minerals. CO₂ late injection caused obvious thermal fluctuation and brought out the result that the montmorillonite of the illite/montmorillonite mixing layer was 15%～30% lower than that of mudstone at the nearby depth. Because of late accumulation of CO₂, acidic environment of reservoir bed lasted for a long time, which promoted dissolution of feldspar and formation of kaolinite. Acidic environment inhibited illite in favor of the transform of other clay minerals into kaolinite. The CO₂-bearing reservoir bed was characterized by high authigenic kaolinites and low illite, which played a decisive role in reservoir quality improvement.