The present invention relates to a method for preparing an imide substituted copolymer resin comprising the steps of: copolymerization by feeding a mixture of an aromatic vinyl monomer and a vinyl cyanide monomer, a mixture of an unsaturated dicarboxylic anhydride monomer and a solvent, an initiator and a chain transfer agent at once to a copolymerization reactor; and imide substitution by continuously feeding the resultant polymerization solution to an imide substitution reactor while continuously feeding a primary amine. The preparation method according to the present invention is capable of continuously preparing an imide substituted copolymer resin having superior heat resistance and excellent fluidity and improving mechanical property and compatibility with ABS resin by inhibiting formation of aromatic vinyl homopolymer.