Provided is a method for preparing a gas separation membrane, the method including forming a porous layer by coating a hydrophilic polymer solution on a porous substrate; and forming an active layer by coating a composition for forming an active layer including a polymer of Chemical Formula 1 on the porous layer, wherein in Chemical Formula 1, n is the number of a repeating unit, and is an integer of 500 to 3,000, and R1 to R5 are the same as or different from each other, and each independently is hydrogen, an alkyl group, or —(C═O)R6, and R6 is an alkyl group, wherein the polymer of Chemical Formula 1 is included in an amount from 1% by weight to 5% by weight based on the composition for forming an active layer, and a gas separation membrane prepared using the same.