Provided is preparation of poly(alkylene carbonate) through alternating copolymerization of carbon dioxide and epoxide. According to the disclosure, by introducing a diepoxide compound to alternating copolymerization of carbon dioxide and epoxide compound using a metal(III) prepared with salen-type ligands containing quaternary ammonium salt as a catalyst, some of the polymer chains may be cross-linked to thus increase an average molecular weight of the copolymer and extend a distribution of molecular weight. A resin prepared according to this method may have high mechanical strength and rheological advantages.