The invention discloses a preparation method of polyacrylonitrile resin, which is characterized in that a catalyst of copper bromide and a ligand of hexamethylenetetramine are dissolved into a monomer acrylonitrile to form a complexing system, a trigger of carbon tetrachloride and an emulsifier of polyoxyethylene laurel ether aqueous solution are added into the system, then a reducing agent of ascorbic acid aqueous solution is added to carry out electron transfer so as to generate atom transfer living radical emulsion polymerization of the catalyst, the remained monomer acrylonitrile is addedafter a certain time of reaction, the action is continued for at least 1 hour, and the product is washed by concentrated hydrochloric acid, demulsified by absolute ethyl alcohol, precipitated, filtered by pumping and dried. By the preparation method of the invention, the polyacrylonitrile resin with narrow molecular weight distribution can be obtained.