An organic zinc catalyst having more uniform and fine particle size and showing more improved activity in a polymerization process for the preparation of a poly(alkylene carbonate) resin and a preparation method thereof, and a method of preparing a poly(alkylene carbonate) resin using the organic zinc catalyst, are provided. The preparation method of the organic zinc catalyst includes the steps of surface-treating a zinc precursor with a dispersant, and reacting the surface-treated zinc precursor with dicarboxylic acid to form a zinc dicarboxylate-based catalyst.