Abstract: Distribution of Mg in melt pool and its effects on mechanical properties of seam were investigated in 5A06 alloy electron beam welding(EBW),and influence of welding parameters on evaporation and loss of Mg element was analyzed.With the deepth in melt pool increasing,containment of Mg grew,indicating that evaporation and loss of Mg reduced,and micro-hardness increased.With increasing of accelerating voltage and beam current,fusion penetration increased,evaporation and loss of Mg decreased.With increasing of welding speed,evaporation and loss of Mg reduce while fusion penetration diminish.In order to minimize evaporation and loss of Mg,the considerable increasing of accelerating voltage,beam current and welding speed is preferred.