Abstract: The numerical simulation on vacuum electron beam deep penetration welding for AZ61 magnesium alloy sheet with 10 mm thickness was studied. In view of the thermal effect about metal steam plasma and keyhole deep penetration during vacuum electron beam welding,the moving heat source model composed by Gauss surface source and double ellipsoid body source was developed. And the characteristics of thermal cycle and weld appearance under different welding processes were studied on the basis of simulation and experiment values. The results of simulation and experiment show that the composite source model can gain the effect of deep penetration of electron beam welding and calculate the temperature distribution and the weld geometry under the action of electron beam source. The results also prove that the model is applicable to the thermal effect simulation on the electron beam welding of AZ61 magnesium alloy sheet.