Abstract: The hydrogeochemical reaction of the high-pressure bentonite in the high-level nuclear waste repository was simulated with the software Phreeqc. A one-dimensional transport model is established in consideration of the cation exchange of K+,Ca2+,Na+,Mg2+ and mineral dissolution and precipitation reactions. The curves of the bentonite chemical components changing over time and space are obtained, which verifies that the decay in swelling capacity is mainly caused by the transformation from Na-montmorillonite into Ca-montmorillonite. The degree of decay in swelling capacity is evaluated. Decay coefficients of the GMZ and MX80 bentonite respectively reach 0.83 and 0.79 in the seawater and 0.91 and 0.90 in the underground water after 10000 years.