The present invention discloses a preparation method of carbon nanotube by decomposing the polymer with hydrotalcite as a catalyst, which belongs to the field of preparation technology of carbon nanotube (CNT). The technical solution of the present invention are as below: firstly, the hydrotalcite with the particle size at nanometer or sub-micron level is prepared, and then is added into the polymer. After the calcination process at high temperature and a treatment with acid, the nano-scale CNT can be obtained. The CNTs prepared by the method supplied in this invention not only have the advantages including high yield, uniform diameter, few structural defects, low impurity content, low cost and simple preparation process, which is suitable for large-scale industrial production, but also can solve the problem of recirculation of waste plastics and utilization of the resource.