Polyacrylonitrile with appropriate molecular weight was prepared by solution copolymerization in the solvent dimethyl sulfoxide using acrylonitrile, itaconic acid and methyl acrylate as copolymerizing monomers and azo disobutyronitrile as the initiator. The effects of polymerization reaction time, reaction temperature, monomer concentration and additive concentration on the quality of the polymerization solution were explored. The results show that the reaction time is 12 to 14 hours, the reaction temperature is 63 ℃ in the early stage and 69 ℃ in the later stage, the mass fraction of the monomer is 21%, the mass fraction of the initiator is 0.4%, and the mass fraction of the terminator is 0.1%, the rotational viscosity, conversion rate, relative molecular mass and its distribution in the obtained polymer solution are appropriate, meeting the production requirements of large-tow carbon fiber.