The invention discloses a high-temperature-resistant cable sheath material and a preparation method thereof. The sheath material comprises the following raw materials in parts by weight: 70-80 of high density polyethylene, 20-30 of ethylene-propylene-diene monomer, 10-15 of maleic anhydride grafted polyethylene, 5-10 of bisphenol A novolac epoxy resin, 8-12 of diphenyl tolyl phosphate, 4-8 of dioctyl maleate, 0.5-1 of di(tert-butylperoxyisopropyl)benzene, 2-3 of p-benzoquinone dioxime, 1-2 of triallyl isocyanurate, 3-5 of paraffin oil, 2-3 of active zinc oxide, 10-15 of magnesium hydroxide, 5-10 of aluminum hydroxide, 4-6 of talcum powder, 8-12 of fly ash, 5-10 of nano bauxite, 2-3 of dibutyltin dilaurate, 1-2 of an anti-aging agent MB, 1-2 of an anti-aging agent TPPD, and 4-6 of a composite filler. The sheath material prepared by the invention not only has good high temperature resistance and can be used in a 125 DEG C working environment for a long time, but also has good flame retardant effect, has the oxygen index of more than 35%, has non halogen and low smoke, and solves the problem of poor heat resistance of a high-flame-retardant sheath material.