Abstract: Distribution of Mg in melt pool and its effect on mechanical properties of weld are investigated in 5A06 aluminium alloy electron beam welding, and the influence of welding parameters on evaporation loss of Mg element is analyzed.The following results are obtained:with the fusion penetration increasing, the content of Mg grows, which means that evaporation loss of Mg reduces, and micro-hardness increases; with increasing of accelerating voltage and beam current, the fusion penetration increases, and the evaporation loss rate of Mg decreases; and with increasing of welding speed, the evaporation loss rate of Mg reduces while fusion penetration diminishes.In order to minimize evaporation loss of Mg, the considerable increasing of accelerating voltage, beam current and welding speed are preferred.