Magnesium in salt mud was extracted by carbon dioxide method. The factors affecting its transformation into light magnesium oxide and the main reaction kinetics were studied. The optimum conditions are as follows：solid-liquid ratio 1∶15，carbon dioxide aeration time 90 min，hydrolysis temperature 90 ℃，hydrolysis time 25 min. Under these conditions，the total magnesium extraction rate is 75%. The reaction order and activation energy of the reaction between magnesium hydroxide and carbon dioxide are obtained by differentiation method，which are 1.7 and 68.12 kJ/mol respectively.