The invention discloses a method for preparing polyacrylonitrile by single electron transfer active free radical polymerization. The method is characterized by dissolving catalyst iron powder and a ligand in a solvent, namely N,N-dimethyl formamide, adding an initiator, namely carbon tetrachloride, into monomers, namely acrylonitrile, repeatedly introducing nitrogen for removing oxygen for at least three times, performing the single electron transfer active free radical polymerization at a certain temperature, washing a product by concentrated hydrochloric acid, settling, performing suction filtration, and drying. With the preparation method disclosed by the invention, the polyacrylonitrile with a molecular weight distribution of 1.10-1.17 can be obtained.