A new method coupling CO2 mineralization and solvent extraction was proposed and investigated to precipitate magnesium from high Mg/Li ratio brine. Organic amine was employed to extract HCl formed during the CO2 mineralization process and to realize continuous conversion of MgCl2 to precipitated MgCO3, thereby removing the magnesium contained in the brine. The effects of brine concentration, extractant, diluents, extractant concentration, phase ratio, and reaction temperature were systematically investigated. A blend of trioctylamine and isoamyl alcohol was used as the extractant and diluent, and the reaction parameters were optimized. The mechanism was proposed, and a stripping process was designed. Under the optimal conditions, the conversion of magnesium was as high as 67.41%, and the Mg/Li ratio of the raffinate co decreased from 20 to 5.4.