Magnesite was successfully synthesized from hydromagnesite by undergoing a phase transition process in monoethylene glycol (MEG) solution. The process can be achieved at 393 K under 0.5 MPa of CO2 in 5 h, and the formed particle size of magnesite ranges from 1 to 10 mu m depending on the synthesis temperature. This study discloses the effect of MEG concentration with adequate amount of sodium chloride (NaCl) or sodium carbonate (Na2CO3) on the magnesite synthesis process, both of which accelerates the mass transfer rate of CO2 during the transition process. The mixed-solvent electrolyte (MSE) model was applied by constructing the phase diagram of hydromagnesite -magnesite in the Mg-Na-Cl-CO3-CO2-MEG-H2O system. Both the experimental and the calculated results unveiled that the addition of MEG reduces the water activity significantly, which allowed for the phase transition to occur. Additionally, rhombic magnesite can be obtained from hydromagnesite in an aqueous solution at a condition that contains 95 wt% of MEG, 1 mol/kgw. NaCl and 0.285 mol/kgw. of Na2CO3.