The present invention relates to carbon nanotubes and a preparation method thereof by using PET. The carbon nanotubes of the present invention are prepared by processes of alcoholysis of PET materials, processes of washing, crushing and calcining unreacted intermediates and so on. By the preparation method of the present invention, multi-walled carbon nanotubes prepared by using waste PET have a good conductivity, and are a structure of top-down array with low aspect ratio. The method of the present invention is not only easy to implement, but also does not need a catalyst, and turns the waste PET into treasure, which solves the problem of environmental pollution caused by the increasingly serious waste PET. FIG. 9.