The invention discloses a preparation method of a polyacrylonitrile spinning solution. The polyacrylonitrile spinning solution is a solution of a main monomer and a comonomer polymer, the main monomer is acrylonitrile with the mole content of 90-99.9%, and the comonomer is itaconic acid mono-methyl ester ammonium compound with the mole content of 0.1-10%. The quality of the polymer solution has direct influence on protofilament performance, and good hydrophily, narrow molecular weight distribution and relatively uniform chain structure are the necessary conditions for obtaining high-performance carbon fibre protofilament. Itaconic acid mono-methyl ester ammonium compound with stronger hydrophily is used for replacing itaconic acid mono-methyl ester as the comonomer, and the hydrophily of the obtained spinning solution is higher than that of the itaconic acid mono-methyl ester, so the problem of loose solidified filament structure caused by increased diffusion rate difference between solvent and nonsolvent in the fibre because the hydrophily of the spinning solution is reduced is avoided. The copolymerization and the polymer chain structure of the invention are relatively easier to control, so the polyacrylonitrile spinning solution with high carbon fibre mechanical properties can be obtained.