There is provided a method for preparing an acrylic copolymer resin for an optical film includes: suspension-polymerizing an acrylic monomer containing a benzene ring, an alkyl(meth)acrylate monomer, and a (meth)acrylic acid monomer to prepare a copolymer; and thermally treating the copolymer at a temperature ranging from 240° C. to 300° C. As the method for preparing an acrylic copolymer of the present invention, a resin having an effectively lowered CTE can be manufactured by inducing the formation of a glutaric anhydride structure by using suspension polymerization that facilitates adjustment of the molecular weight of a resin.