Abstract: Utilizing the effect of temperature extremum of powders molten pool caused by the interactive effect of electron beam(EB) with metal powders,the measurement methods of EB focus are studied.The effecting factors on measurement methods are analyzed.During electron beam processing,the rules of metal powders molten pool temperature changing with focusing current are discovered.The peak temperature of metal powder pool corresponds to the critical transferring point of energy density in the dynamic focusing electron beam processing.Based on the temperature characteristics of critical energy density,the measuring method of dynamic focal spot of electron beam is put forward in the paper.The method of measuring dynamic focal spot of electron beam will provide a new prospect for 3D scanning prototyping through changing focus position of EB.