The present invention relates to an electrolytic machining method. For increasing size precision of electrolytic machining method, a metallic mask layer is formed on the surface of a workpiece whose material has high conductivity or volume electrochemical equivalent, whereby the metallic mask layer can be used as a sacrificial layer of electrolytic machining and simultaneously protects the non-machining region of the workpiece so as to reduce lateral machining of the workpiece Consequently, the size precision of electrolytic machining is enhanced. In addition, the feasibility of electrolytically machining a miniature interval between two machined structures is increased as well. In addition, the present invention provides a semifinished electrolytic workpiece, comprising a workpiece and a metallic mask layer formed on the surface of the workpiece. The conductivity or volume electrochemical equivalent of the metallic mask layer is smaller than that of the workpiece.