The present disclosure relates to an economical method of preparing a resin composition including a polyalkylene carbonate with improved thermal stability and processability. More specifically, the method of preparing a resin composition includes the steps of polymerizing carbon dioxide and an epoxide compound in the presence of a zinc-based catalyst and a solvent, recovering monomers, removing the catalyst and recovering raw materials, solution-blending with a thermostable resin to improve the thermal stability and processability, and removing the solvent and byproducts from the reaction mixture by using an agitated flash drum and an extrusion or kneader-type devolatilizer.