There are a lot of deep holes in aero-engine parts, which are not only small aperture, large aspect ratio, but also require no recast layer on the surface of the hole. These characteristics bring a lot of difficulties to the machining of the hole. Electrochemical machining (ECM) is an effective machining method. In view of the difficulties in preparing the side wall insulating layer of cathode used in ECM of small holes with extra large aspect ratio and the problems that it is easy to be damaged in the process of use, a variety of materials and methods for preparing the insulating layer were tried, and the performance stability during the processing was experimentally studied. Finally, a new preparation method of multilayer composite insulating layer based on fused silica capillary column was proposed, and the tube cathode was fabricated by this method. A deep hole with a diameter of 1.1 mm and a depth to diameter ratio of 181 was successfully fabricated on stainless steel material.