The invention relates to a preparation method of a carbon-nanotube-enhanced magnesium-based composite material. The method comprises the steps that: carbon nanotubes are subjected to surface modification, such that a uniform and compact Ni-P alloy layer is formed on the surface of the carbon nanotubes; the modified carbon nanotubes are mixed with powders of elements such as magnesium, aluminum, and zinc, such that a mixed raw material is obtained; the mixed raw material is mixed with ceramic balls, such that mixed powder is obtained; the mixed powder is placed in a mold, and is subjected to bidirectional cold-pressing under room temperature; the composite material obtained by cold-pressing is subjected to vacuum sintering with the mold; and the composite material obtained by vacuum sintering is subjected to hot extrusion. With the method provided by the invention, the carbon-nanotube-enhanced magnesium-based composite material with high performance, light weight, and high strength can be prepared. The combination of the enhancing phase and the substrate is good. The material is advantaged in relatively high specific strength, high specific rigidity, high thermal conductivity, excellent machining performance, and the like. The composite material has good application prospect in industrial fields such as aeronautics and astronautics, automotives, 3C industries, sports and entertainments, and the like.