The invention discloses an aging-resistant insulating sheath material for a transformer and a preparation method thereof. The aging-resistant insulating sheath material is prepared from 50-70 weight parts of methyl vinyl silicone rubber, 20-30 weight parts of butadiene rubber, 15-20 weight parts of low density polyethylene, 20-30 weight parts of precipitated silica, 3-5 weight parts of zinc oxide, 1-2 weight parts of stearic acid, 2-3 weight parts of dicumyl peroxide, 1-2 weight parts of sulfur, 1-2 weight parts of an antiager OD, 1-2 weight parts of a promoter TMTD, 4-8 weight parts of diphenylsilanediol, 5-10 weight parts of epoxidized soybean oil, 8-12 weight parts of triethyl O-acetylcitrate, 3-6 weight parts of hydroxyl silicone oil, 5-10 weight parts of zinc borate, 4-8 weight parts of antimonous oxide, 10-15 weight parts of nanometer potassium feldspar, 5-10 weight parts of nanometer aluminum oxide, and 3-6 weight parts of a composite filling material. The aging-resistant insulating sheath material has excellent aging resistance, excellent acid and alkali resistance, excellent smoke resistance, excellent ozone resistance, excellent weather resistance, good insulating properties, and good physical and mechanical properties, can be used out of doors for a long time, can resist strong acid, strong alkali and salt spray, can be exposed to the sun and rain, does not deform and crack, and is durable.