The invention relates to a method for preparing nanometer carbon fiber, in particular to a method for preparing the nanometer carbon fiber by taking tobacco as raw materials. The method for preparingthe nanometer carbon fiber by taking the tobacco as the raw materials comprises the following steps of (1) preparing a metal sample with catalytic activity; (2) putting the metal sample with the catalytic activity obtained in the step (1) into an acid solution; performing ultrasonic treatment for 0.5 to 3h; filtering the solution; rinsing filter slag by ethanol or de-ionized water until the pH value is 7.0; then, collecting the filter slag; performing drying to obtain the nanometer carbon fiber. The method for preparing the nanometer carbon fiber by taking the tobacco as the raw materials provided by the invention has the advantages that the raw material price is low; the application of tobacco to other new industries is facilitated; the preparation process of the carbon nanometer fiber prepared by the method is simple and safe.