The invention relates to a tool cathode and a method for improving electrolysis milling machining bottom face flatness, and belongs to the field of electrolysis milling composite machining. The side wall of the tool cathode is provided with an electric conduction abrasive particle layer and an insulting abrasive particle layer, and insulation of a bottom face is achieved. In the working process, abottom insulating layer reduces electric current density of a bottom face central area, the side wall insulting abrasive particle layer can reduce the electric current density of a bottom face edge area, the problem that the bottom face electric current density difference is large in the electrolysis milling machining process is solved, and the electrolysis milling machining bottom face flatnessis further improved.