Electron beam welding (EBW) is currently the most advanced jointed technology because of its superior welding precision and larger depth-width ratio than other conjunctive methods. The high vacuum environment of EBW is favorable for the welding of magnesium alloys to prevent the active metals from oxidation. In this study, a home-made 11mm-thick AZ91D extrusive plate was used. By operating and comparing the changeable welding parameters, the optimum condition has been determined as 113mA, 40kV, 73.3mm/s and focal position at bottom. 82% and 89% of the matrix material strength were obtained by stress and non-stress concentration weldments, respectively. Under other worse parameters, the reduction of the strength of EBW for AZ91D-F resulted from four factors, which were undercuts, heat affected zone (HAZ), hot cracking, and cavities.