Abstract: With studying conduction current of workpiece, the rules between average ratio of conduction current of workpiece and electron beam current are discovered. The peak of average ratio of conduction current corresponds to the transferring point of critical penetration in the dynamic process of electron beam welding. By the characteristics of critical penetration current, the focusing states of electron beam influenced by metal vapor and transmitting in drift space are discussed. The concept of dynamic focal spot of electron beam is put forward. The method of meas ureing dynamic focal spot of electron beam is invented, through varying focus-extremum of critical penetrating current.