The invention discloses a high-temperature-resistant thermoplastic elastomer and a preparation method thereof. The high-temperature-resistant thermoplastic elastomer is prepared from the following raw materials in parts by weight: 15-25 parts of cis-1,4-polybutadiene rubber, 20-25 parts of acrylate rubber, 5-7 parts of bisphenol A novolac epoxy resin, 10-12 parts of diphenylmethane bismaleimide, 8-12 parts of polyethylene, 5-8 parts of paraffin oil, 3-5 parts of polyethyleneglycol, 4-6 parts of dibasic lead phosphite, 4-6 parts of N,N-diacetyladipyldiamido hydrazine, 1-4 parts of dibutyltin dilaurate, 1-5 parts of citric acid, 3-6 parts of calcium carbonate, 2-4 parts of mullite powder, 8-15 parts of fly ash and 10-15 parts of modified composite filler. The high-temperature-resistant thermoplastic elastomer has the advantages of excellent aging resistance, excellent high-temperature resistance, favorable flame-retardant effect and high oxygen index (up to 33% or above), and can be used in a high-temperature environment for a long time without the phenomena of cracking, bending, shrinkage and the like.