The invention discloses modified boehmite for heatproof flame retardant cable materials and a preparation method thereof, and the modified boehmite is made from the following raw materials by weight: 100 parts of boehmite, 15-25 parts of expanded graphite powder, 3-6 parts of triglycidyl isocyanurate, 20-30 parts of nano black stone, 10-15 parts of ceramic beads, 10-15 parts of butyl benzyl phthalate, 2-3 parts of 4-amion-2,2,6,6-tetramentylniperidine, 5-10 parts of phenyl isopropylphenyl phosphate, 1.5-2.5 parts of 2-hydroxy magnesium benzoate, 2-3 parts of p-hydroxy benzoic acid hydrazide, 4-6 parts of behenamide, 0.5-1.5 parts of resorcinol monobenzoate, 1-2 parts of dilauryl thiodipropionate and 3-5 parts of an auxiliary. The modified boehmite has the advantages of high in acid resistant, flame retardant and smoke suppression functions, good in filling effect, free of secondary pollution after combustion, low cost, and the like, not only can improve the flame retardant properties of the cable materials, but also can improve the heat resistance, corrosion resistance, dielectric property and mechanical strength of the cable materials.