The invention provides a formula and a preparation method of an insulating cable jacket with a long service life. The formula of the insulating cable jacket with the long service life is prepared from the following components in parts by weight: 200 parts of an ethylene-octene copolymer, 15 to 36 parts of low-density polyethylene, 18 to 50 parts of ethylene propylene rubber, 20 to 30 parts of an additive, 2 to 10 parts of mica, 5 to 9 parts of asbestos, 3 to 8 parts of marble powder and 5 to 20 parts of insulating paint. The cable jacket prepared by the formula provided by the invention has a high temperature-resisting grade, good electrical properties, excellent mechanical properties, a soft texture and less charge accumulation in an electron irradiation process and is suitable for manufacturing middle-voltage and low-voltage cables.