Mg?Al alloy was prepared using electrochemical co-deposition method with AlCl3?NaCl double salt as Al source and molten salt prepared by the dehydration of MgCl2?6H2O as the electrolyte. The results showed that the AlCl3?NaCl double salt was relatively stable at high temperature when AlCl3 molar fraction was less than 0.4. The volatile loss of AlCl3 could be effectively reduced when using AlCl3?NaCl double salt as Al source to feed into molten system of high temperature. When the mole fraction of AlCl3 in the double salts was less than 0.5, the usage efficiency of AlCl3 was high in the process of preparation Mg?Al alloy, and Al content in Mg?Al alloy was increased by 9.5% at least compare with those alloys prepared by using AlCl3 as Al source, the deviation from theoretical value of Al content in Mg?Al alloy reduced. Al content in Mg?Al alloy increased linearly with the increase of the AlCl3 content in the molten salt, and the Mg?13%Al alloy has relatively uniform element distribution at macro level, and the grain size of alloy was 50?100 ?m.