Montmorillonite is an important clay mineral in sedimentary rocks. In order to explore the impact of CO₂ injection on montmorillonite and clarify the chemical behavior of montmorillonite in CO₂ aqueous solution，the state of montmorillonite reacting with CO₂ for 7 days and 30 days at 10 MPa and 60 ℃ was systematically evaluated by means of XRD，XRF and ICP，The changes of solid elements，crystal structure and ion concentration in reaction solution before and after montmorillonite powder reaction were compared. Combined with the structural characteristics of the original montmorillonite，the change mechanism of montmorillonite in acid was clarified. The results showed that the concentrations of Ca²⁺，Mg²⁺and Al³⁺ in the liquid phase first increased and then decreased after the reaction of montmorillonite with CO₂. The concentration of Si⁴⁺ increased first and then tended to be stable. The crystal surface corresponding to d（001）in the solid phase was destroyed after the dissolution reaction. Compared with Al³⁺ and Si⁴⁺， the variation range of Mg²⁺，Ca²⁺ and Na+ in the reaction process was relatively large. On the one hand，due to the large cation exchange capacity of montmorillonite，the active cations with large radius such as interlayer Na⁺ ，Mg²⁺ ，Ca²⁺ were easy to be replaced by H+ with small radius；on the other hand，the interlayer intermolecular force of montmorillonite was van der Waals force，which was weak relative to electrostatic force，and the structure was easy to be damaged. Under acidic conditions，the stability of silica tetrahedron was slightly stronger than that of alumina octahedron，but amorphous Si in montmorillonite was easy to dissolve，which made the change range of Si greater than that of Al.