Abstract: The keyhole thermal effect of vacuum electron beam welding for magnesium alloy was analyzed.In view of the thermal effect of high-temperature metal vapor and the deep-penetration effect of keyhole during welding,a composite heat source model applying to magnesium alloy,which was made up of Gaussian surface source and conebody source,was developed.By the different valuation to the power coefficients of Gaussian surface source and cone body source,the various focus states of electron beam were simulated to get the welding temperature fields.And then,the simulation results were favorable for the shape calculation to keyholes and welds.The experimental results show that the simulation results have good consistency with the experiment welds.