The invention discloses a preparation method of multifunctional polyurethane elastic fibers, and belongs to the technical field of high polymer material preparation. The method is particularly suitable for high-speed spinning of spandex fibers, the production efficiency of the spandex fibers is improved, and meanwhile, the spandex fibers have the characteristics of being high in elongation and easy to dye. The method comprises the following steps: (1) uniformly mixing a solvent, N-methyldiethanolamine and polyether polyol; (2) adding diisocyanate MDI based on a set -NCO/-OH ratio, and pre-polymerizing to obtain a prepolymer solution; (3) adding a mixed amine solution of a chain extender and a chain terminator into the prepolymer solution obtained in step (2), and polymerizing to obtain a polyurethane solution; (4) adding a plurality of additives into the polyurethane solution, and fully uniformly mixing to obtain a polyurethane spinning stock solution; and (5) ageing the prepared polyurethane spinning stock solution, and spinning in a dry manner to obtain the polyurethane elastic fibers. The preparation method has the advantage of high production efficiency; and the spandex fibershave the characteristics of being high in elongation, easy to dye, and easy to size.