Abstract: The characteristics of heat source for vacuum electron beam welding of deep penetration were analyzed.On the basis of the gaussian descending for heat source radius and exponential increasing for power density of heat source along the depth,a mathematical model of whirling gaussian body source with increasing peak power was established.Under the level of focusing and defocusing,the power-density distribution for special thermal effect of vacuum electron beam welding was calculated by this model.The results of these calculations showed that the power-density distribution of the heat source model was consistent with the thermal characteristic of electron beam source based on the idealized state.And this model was applicable to the thermal-effect simulation of vacuum electron beam welding at different focus levels.