The invention provides a preparation method of a fir nano fiber. The preparation method comprises the following steps: 1, boiling fir fibers in a boiling liquid to prepare paper pulp; 2, extracting the fir fibers, 3, preparing sodium chlorite solution, adding polyethylene friction blocks into the sodium chlorite solution, treating the fir fibers by the sodium chlorite solution; 4, treating the fir fibers with alkali solution to obtain purified cellulose; 5, preparing iron-ruthenium composite nano-particles; 6, preparing the iron-ruthenium composite nano-particles; 6, subjecting the purified cellulose to catalytic oxidation in the catalytic system comprising TEMPO, the composite nano-particles, sodium bromide and sodium hypochlorite; 7, carrying out a microwave heating reaction on the obtained cellulose suspension liquid under the condition of alternating magnetic field, subjecting the reaction product to washing and drying in vacuum to obtain the absolute dry oxidized cellulose fiber; 8 freeze-drying the suspension liquid to obtain a final product. The method disclosed by the invention is specially used for preparing the fir nano fibers, and the prepared fir nano fibers are very uniformly dispersed, and the nano fibers are low in mutual aggregation degree, high in production efficiency and good in product performance.