There is provided a method for preparing a low-molecular weight poly(alkylene carbonate) of which the molecular weight and chain shape are precisely controlled, by introducing a phosphorous compound having a hydroxyl group as a chain transfer agent in order to regulate the molecular weight, in alternating copolymerizing an epoxide compound and carbon dioxide by using trivalent metal complex prepared from a Salen type ligand containing a quaternary ammonium salt, and a polymer prepared by the method.Since poly(alkylene carbonate) prepared according to the present invention includes a phosphate or phosphonate group in the polymer chain, it has flame-retarding property.