Industrial experiments were carried out for Mg production by aluminothermic process in industrial retorts and the factors affecting the reduction efficiency were analyzed. The results show that the main factors reducing the reduction ratio are oxidation and combustion of crystallized magnesium and uneven mixing of raw materials. The latter could result in raw material regions with low Al concentration and MgO redundance, which can promote the formation of 12CaO·7Al₂O₃ and CaO·Al₂O₃. For raw material regions with higher Al concentration, both MgO and CaO can be reduced to form the Mg₂Ca phase. Radiation and chemical reaction heat are the key factors affecting the reaction rate. Increasing the heating temperature can rapidly increase the bed temperature and obtain sufficient reaction ratio. The higher the magnesium content in the pellets is, the longer the reduction time is required.