The invention relates to a preparation method of a binary polyacrylonitrile carbon fiber precursor. The method comprises the following steps: mixing acrylonitrile, a second comonomer beta-itaconic acid ester amide and a solvent in the presence of nitrogen, and adding an initiator; reacting for 22-38 hours under the condition of 35-75 DEG C, so as to obtain an acrylonitrile polymer; then filtering, taking off and defoaming, so as to prepare a spinning solution; spinning the spinning solution by virtue of a dry-jet wet process, stretching, oiling, drying, densifying, drawing by hot steam, and thermoforming, so as to obtain the binary polyacrylonitrile carbon fiber precursor. By adopting the preparation method, double functional group beta-itaconic acid ester amide is adopted as a comonomer, and a polyacrylonitrile macromolecule for the carbon fiber precursor is prepared by binary copolymerization and substituted ternary copolymerization. Thus, the dosage of the comonomer is reduced, meanwhile, the structural regularity of the polyacrylonitrile macromolecule is improved, and the prepared carbon fiber precursor is even in structure, small in fineness and high in strength.