The present invention relates to a preparation method of polyurethaneurea elastic fiber, particularly a preparation method of high heat-resistant elastic fiber which retains unique physical properties of elastic fiber under the high temperature process. The polyurethaneurea elastic fiber of the present invention is characteristically prepared without using 1 functional mono-alcohol generally used to regulate the reaction speed and side-reaction during the prepolymer reaction to produce elastic fiber having excellent heat-resistance particularly for the dyeing processing or re-dyeing of polyester but using ethylenediamine alone as a chain extender for the polymerization. To reduce gel formation in polymer after the secondary polymerization and to regulate change of viscosity in the course of process, the ratio of the chain extender amine to the terminal group of prepolymer of the first polymerization is increased.