The actuality of lettering technology on workpiece's surface is introduced from domestic and abroad ,and the principle of using EDM to carve characters on large-scale workpiece's surface are analysed at economical and technologic point of view. The technologic scheme is put forward according to the principle of EDM and the character of EDM lettering on large scale workpiece's surface,which is based on headstock and electrical source control tank of minitype EDM tool. The key technologies, the whole makeup,the motion between electrode and workpiece at three coordinates, precise orientation of electrode and cycle system of work liquid are emphatically studied. Finally,an special EDM lettering tool of lager scale roller for a large scale iron & steel enterprise is briefly described. The application indicates that the technologic scheme is economical and useful,and the EDM lettering tool is stable and reliable.