The invention provides a preparation method of a bacterial cellulose-derived carbon nanofiber aerogel. The preparation method comprises the following steps: (S1) soaking bacterial cellulose into a solution containing inorganic ammonium salt, and carrying out freeze drying, so as to obtain bacterial cellulose aerogel; and (S2) carrying out high-temperature pyrolysis on the bacterial cellulose aerogel, so as to obtain the bacterial cellulose-derived carbon nanofiber aerogel. Compared with the prior art, the preparation method has the advantages that inorganic ammonium salt is added as a catalyst to change a fiber pyrolysis route of the bacterial cellulose, so that dehydration is effectively promoted, the carbon-oxygen ratios in dissipated gas-state and liquid-state products are decreased, more carbons are remained in a solid-stage product, the carbon residue content is increased after the pyrolysis of the bacterial cellulose, finally, the yield of the carbon nanofiber aerogel is increased, and the product is relatively low in density, relatively large in specific surface area and good conductivity and mechanical property; and furthermore, by adjusting the solution concentration of inorganic ammonium salt, carbon nanofiber aerogels with different densities and mechanical properties are prepared.