An experiment on API G-level(American Petroleum Institute) cement is conducted after curing under a supercritical carbon dioxide environment. First, cement paste is prepared to generate a uniaxial compressive specimen, after which the specimen is exposed to the supercritical carbon dioxide environment (temperature = 70 °C; pressure = 20 MPa) for curing at different numbers of days (7~84 days). Subsequently, the physical and chemical changes in the cement are simulated at 1500~2000 m below the injection well during CO2 sequestration. Results show that the uniaxial compressive strength of the specimen decreases as the number of curing days increases, indicating that the specimen sustains considerable damage when cured under humid environments. This result also implies a declining trend in the longitudinal and transverse waves of the cured specimen. On the basis of the analytical results for the materials, we determine that carbon dioxide reacts with the calcium hydroxide, water, and calcium silicate in the cement. The carbon dioxide is then converted into calcium carbonate and results in different degrees of carbonization depending on the number of curing days.