Electron Beam Melting (EBM) shows excellent performance in making customized, net shaped metal parts, while to fully explore the potential of making large parts (one direction larger than 200mm along the manufacturing platform), the current EBM machines face an obstacle, that the electron beam defocuses near the perimeter as it scans over a broad flat area. Experiments show that metallurgical bonding in defocused zone is poor and the precision is lower, limiting the potential of EBM technology. In this paper, a strategy of digitized dynamic focus control in EBM is proposed, and the focus length is so adjusted according to beam path in real time that beam focuses as designed over the broad, flat area (200mm × 200mm).