Abstract: Disclosed a polyurethane elastic fiber and a method of producing the polyurethane elastic fiber. The method of producing the polyurethane elastic fiber is characterized in that polyol with high molecular weight and diisocyanate with excessive amount are mixed in a condition of a designated shear rate and prepolymerized to produce a prepolymer, the prepolymer is reacted with the chain extender and the chain terminator to produce a polymer, and an additive is added to the polymer and the final polymer is spun. The method of producing the polyurethane elastic fiber according to the present invention improves the stability of the polymer, has an excellent spinnability even in high-speed spinning, and remarkably reduces the generation of wave yarns. The polyurethane elastic fiber of the present invention is excellent in heat resistance, thermosetting efficiency and coherence strength between the monofilaments.