Provided is a method of preparing an acrylonitrile-based copolymer for a carbon fiber, which comprises: preparing a reaction solution including a monomer mixture comprising a sulfonate-based monomer represented by Chemical Formula 1, a carboxylic acid-based monomer, and an acrylonitrile-based monomer and an organic solvent; and subjecting the reaction solution to polymerization, wherein the monomer mixture comprises the sulfonate-based monomer represented by Chemical Formula 1 at 0.55 to 1.6 mol%.
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