The invention relates to a heat-resistant polyurethane elastic fiber and a preparation method thereof. The preparation method comprises the following preparation steps of firstly mixing diisocyanate with hydroxyl-terminated polyether, then carrying out a prepolymerization reaction to obtain a prepolymer 1, meanwhile, mixing the diisocyanate and amino-terminated polyether with an alkali metal, andthen carrying out the prepolymerization reaction to obtain a prepolymer 2; dissolving respectively to obtain a prepolymer 1 solution and a prepolymer 2 solution, mixing the two solutions, then addinga chain extender and a terminator to carry out a chain extension reaction to obtain a polyurethane solution; finally, spinning to obtain the heat-resistant polyurethane elastic fiber. A molecular mainchain of a final product includes two hard-segment structures at the same time; the product also has excellent heat-resistant performance while the excellent elastic-force performance is ensured, andhas huge economic value. The preparation method of the heat-resistant polyurethane elastic fiber is simple in process and convenient for large-scale production and is safe and environment-friendly and the heat-resistant polyurethane elastic fiber has terrific popularization value.