The invention discloses a urethane elastic fiber with high temperature resistance and high elasticity and a preparation method thereof, the materials of the urethane elastic fiber comprise: polyether diols, diisocyanate, butanol, amine admixture, anti-oxidant, reelability agent, ultraviolet absorber, yellowing resistant agent and dimethyl acetamide; the preparation method of the urethane elastic fiber is that: a prepolymer is obtained through prepolymerization reation, and macromolecule polymer is also obtained through continuous chain extension reactions, then spinning solution is formed by adding different auxiliary agents; finally winding and dry spinning through round spinning channel are performed to obtain a high temperature high elastic urethane fiber spinning can which has the advantages of high temperature resistance, high elasticity, excellent reelability, outstanding yarn evenness, etc, and the preparation method can realize the continuous process of chain extension reactions and addition of auxiliary agents with continuous and stable production process and high product evenness and can completely meet the requirements of all kinds of novel fabrics and high grade fabrics.