A new method of preparing Mg-Li alloys by vacuum carbothemic reduction is proposed. The thermodynamics of the reduction reactions were analyzed and the equations of the reduction reactions, the Gibbs free energy and critical temperature of reactions also have been studied. The results show that vacuum carbothermic reduction producing Mg-Li alloys had the thermodynamics feasibility. Under the condition of the other factors unchanged, with the increase of vacuum degree and reaction temperature, the Gibbs free energy of reduction reaction reduced. Under the same vacuum degree condition, the critical temperature of vacuum carbothermic reduction producing Mg-Li alloys was lower than producing metallic Mg or metallic Li, and reaction was more easily. When the vacuum degree was 10Pa and the relative proportion of Li2O was 0.1, the critical temperature of vacuum carbothermic reduction producing Mg-Li alloys was 1345K. Under the reaction temperature (about 1473K) and vacuum degree (about 13.3Pa) of conventional Pidgeon process(producing metallic magnesium by vacuum silicothermic reduction) conditions, no matter what the relative proportions of magnesium oxide and lithium oxide in the hybrid system, vacuum carbothermic reduction producing Mg-Li alloys had the thermodynamics feasibility.