A plasticizing agent and a composition for fabricating a polyacrylonitrile-based fiber precursor and a fabrication method of a polyacrylonitrile-based carbon fiber are provided. The plasticizing agent includes a copolymer represented by formula (I) or a derivative of formula (I): wherein R is methyl or ethyl, z≧0.5 mol %, and y=99.5-85.0 mol %. The plasticizing agent has an intrinsic viscosity of between 0.20-0.40 dL/g. The composition for fabricating the polyacrylonitrile-based fiber precursor includes the plasticizing agent and a polyacrylonitrile-based copolymer having an intrinsic viscosity of between 0.41-0.75 dL/g.