The invention discloses black polyurethane-urea elastic fiber added with a carbon nanotube and a preparation method thereof. In the elastic fiber, the carbon nanotube is used as black packing of spandex; after the carboxylation and amination treatment on the surface, the carbon nanotube can be uniformly dispersed in spandex and forms an effective chemical bonding effect with polymer molecules; the addition amount of the carbon nanotube is 0.01-1% of the mass of the polyurethane-urea elastic fiber. In the invention, the carbon nanotube is used as a coloring agent of black spandex for the first time, and the problem of difficult dispersion of the carbon nanotube in spandex and the shortcoming of weak bonding effect of a two-phase interface are avoided. The black spandex prepared by the method has good black effect and color fastness, high mechanical strength, high elongation, low volume resistivity and excellent antistatic effect; moreover, the black spandex has excellent heat resistance and high temperature resistance and can endure long-time high-temperature boiling while the retention rate of mechanical strength is high.