Abstract: A simple cylindrical physical model of keyhole was developed according to the characteristic of deep-penetration effect in vacuum electron beam welding.An analysis model of heat transfer on the interface of vapor phase and front keyhole wall was proposed to the temperature calculation on the basis of heat transfer theory.The evaporation of the primary elements,which are Mg,Al,Zn and Mn in AZ series magnesium alloy,can be analyzed by the model,as well as the influence of keyhole radius varying to the temperature on the vapor-solid interface of the front keyhole wall.The calculation result shows that Mg and Zn are vulnerable to vaporize loss during the vacuum electron beam welding on AZ series magnesium alloy,and the evaporation of Mg occurs earlier than the Zn.The smaller keyhole radius will significantly increase the temperature of the front keyhole wall,which has a great influence on the evaporation effect of the elements.