Provided is preparation of poly(alkylene carbonate) by alternating copolymerization of carbon dioxide and epoxide. To be specific, provided are a method for preparing block or graft copolymers of the polymer compound and poly(alkylene carbonate) by alternating-copolymerization of an epoxide compound and carbon dioxide by using a metal (III) compound prepared from salen-type ligand with a quaternary ammonium salt as a catalyst in the presence of a polymer compound having a functional group of hydroxyl or carboxylic acid, and block or graft copolymers prepared by the method.