The invention discloses a high and low temperature-resistant insulating sheath material for a transformer and a preparation method thereof. The high and low temperature-resistant insulating sheath material is prepared from 70-90 weight parts of fluorosilicone rubber, 20-30 weight parts of SG-3-type PVC, 10-15 weight parts of dioctyl maleate, 5-10 weight parts of nylon acid diisobutyl ester, 15-20 weight parts of magnesium hydroxide, 10-15 weight parts of ammonium polyphosphate, 4-8 weight parts of molybdenum trioxide, 30-40 weight parts of fumed silica, 4-6 weight parts of zinc oxide, 2-3 weight parts of stearic acid, 1-2 weight parts of nano-cerium oxide, 1-2 weight parts of bis(P,P-bis-ethylhexyl diphosphato)ethanediolato titanate, 3-6 weight parts of oxidized polyethylene wax, 10-15 weight parts of brucite powder, 15-20 weight parts of nano-barite, 8-12 weight parts of nano-wollastonite and 4-6 weight parts of a composite filling material. The high and low temperature-resistant insulating sheath material has excellent high and low temperature resistance, good insulating properties, good physical and mechanical properties, good weathering aging resistance, acid and alkali resistance, radiation resistance and oil resistance, can be used in a temperature range of -60-180 DEG C for a long time, does not deform, embrittle and crack, and is durable.