The invention discloses a method for preparing a multiwalled carbon nanotube through microwave enhanced fast pyrolysis of biomass and / or carbonaceous organic waste. The method comprises the following steps: biomass, carbonaceous organic waste, a mixture of the biomass and carbonaceous organic waste or a mixture of the biomass and carbonaceous organic waste uniformly mixed with a microwave absorbent is placed in a reaction vessel in a microwave cavity; an inert gas is introduced into the reaction vessel until an oxygen-free environment is formed; the microwave input power is adjusted to be higher than 500w, and the reaction vessel is heated to 400-1,500 DEG C for a pyrolytic reaction; the multiwalled carbon nanotube is obtained after the reaction. The method for preparing the multiwalled carbon nanotube is cheap and easy, the prepared carbon nanotube is curly, has the mean diameter of 3 nm-200 nm, can be applied to the fields of composites, electrode materials, catalyst preparation and the like and has a broad application prospect.