In the process of CO2 geological sequestration, there is a risk that CO2 will migrate to and dissolve in groundwater, resulting in a series of physical and chemical reactions, with influence on chemical compositions of groundwater. In this paper, numerical simulations are conducted to study the influence of CO2 migration on the chemical composition of groundwater. The modeling results show that when CO2 migrates to the groundwater, gaseous CO2 will migrate upward and diffuse laterally under the effect of buoyancy. The acidity of groundwater is enhanced and the pH is significantly reduced, leading to dissolution of calcite, illite, oligoclase, K-feldspar, chlorite and hematite, and precipitation of quartz, kaolinite, smectite-Na, siderite and smectite-Ca. The porosity of the formation increases. The concentrations of K+ and Fe2+ in groundwater gradually increase, the concentrations of Ca2+ and HCO3− basically remain unchanged after increasing to a certain value, the concentrations of Mg2+ and AlO2− first increase and then decrease, and the concentrations of Na+, Cl− and SO42− first decrease slightly and then gradually increase, but the change in concentration is small. Therefore, Fe2+, Mg2+, Ca2+, K+ and pH can be used as important monitoring indicators of whether CO2 migrates to groundwater during geological storage.