A method for preparing a membrane for flue gas separation comprising a crosslinked, thermally rearranged poly(benzoxazole-co-imide) copolymer having a repeat unit represented by Chemical Formula 2 comprising: a) obtaining a polyamic acid solution by reacting an acid dianhydride, an o-hydroxydiamine and an aromatic diamine represented by the General Formula 4 H 2 N-Ar 2 -NH 2 , and 3,5-diaminobenzoic acid, as comonomers, and synthesizing an o-hydroxypolyimide copolymer having carboxylic acid groups through azeotropic thermal imidization; b) preparing a membrane by dissolving the o-hydroxypolyimide copolymer having carboxylic acid groups synthesized in a) in an organic solvent and casting the same; and c) heat-treating the membrane obtained in b), wherein Ar 1 is an aromatic ring group selected from a substituted or unsubstituted tetravalent C 6 -C 24 arylene group and a substituted or unsubstituted tetravalent C 4 -C 24 heterocyclic ring group, wherein the aromatic ring group exists on its own, forms a condensed ring with each other, or is linked with each other by a single bond, O, S, CO, SO 2 , Si(CH 3 ) 2 , (CH 2 ) p (1 ≤ p ≤ 10), (CF 2 ) q (1 ≤ q ≤ 10), C(CH 3 ) 2 , C(CF 3 ) 2 or CO-NH, Q is a single bond, O, S, CO, SO 2 , Si(CH 3 ) 2 , (CH 2 ) p (1 ≤ p ≤ 10), (CF 2 ) q (1 ≤ q ≤ 10), C(CH 3 ) 2 , C(CF 3 ) 2 , CO-NH, C(CH 3 )(CF 3 ) or a substituted or unsubstituted phenylene group, Ar 2 is an aromatic ring group selected from a substituted or unsubstituted divalent C 6 -C 24 arylene group and a substituted or unsubstituted divalent C 4 -C 24 heterocyclic ring group, wherein the aromatic ring group exists on its own, forms a condensed ring with each other, or is linked with each other by a single bond, O, S, CO, SO 2 , Si(CH 3 ) 2 , (CH 2 ) p (1 ≤ p ≤ 10), (CF 2 ) q (1 ≤ q ≤ 10), C(CH 3 ) 2 , C(CF 3 ) 2 or CO-NH, and x, y and z are molar fractions in the repeat unit, satisfying x < 0.8 and x + y + z = 1, except for the case where x, y or z is 0. a) obtaining a polyamic acid solution by reacting an acid dianhydride, an o-hydroxydiamine and an aromatic diamine represented by the General Formula 4 H 2 N-Ar 2 -NH 2 , and 3,5-diaminobenzoic acid, as comonomers, and synthesizing an o-hydroxypolyimide copolymer having carboxylic acid groups through azeotropic thermal imidization; b) preparing a membrane by dissolving the o-hydroxypolyimide copolymer having carboxylic acid groups synthesized in a) in an organic solvent and casting the same; and c) heat-treating the membrane obtained in b), wherein Ar 1 is an aromatic ring group selected from a substituted or unsubstituted tetravalent C 6 -C 24 arylene group and a substituted or unsubstituted tetravalent C 4 -C 24 heterocyclic ring group, wherein the aromatic ring group exists on its own, forms a condensed ring with each other, or is linked with each other by a single bond, O, S, CO, SO 2 , Si(CH 3 ) 2 , (CH 2 ) p (1 ≤ p ≤ 10), (CF 2 ) q (1 ≤ q ≤ 10), C(CH 3 ) 2 , C(CF 3 ) 2 or CO-NH, Q is a single bond, O, S, CO, SO 2 , Si(CH 3 ) 2 , (CH 2 ) p (1 ≤ p ≤ 10), (CF 2 ) q (1 ≤ q ≤ 10), C(CH 3 ) 2 , C(CF 3 ) 2 , CO-NH, C(CH 3 )(CF 3 ) or a substituted or unsubstituted phenylene group, Ar 2 is an aromatic ring group selected from a substituted or unsubstituted divalent C 6 -C 24 arylene group and a substituted or unsubstituted divalent C 4 -C 24 heterocyclic ring group, wherein the aromatic ring group exists on its own, forms a condensed ring with each other, or is linked with each other by a single bond, O, S, CO, SO 2 , Si(CH 3 ) 2 , (CH 2 ) p (1 ≤ p ≤ 10), (CF 2 ) q (1 ≤ q ≤ 10), C(CH 3 ) 2 , C(CF 3 ) 2 or CO-NH, and x, y and z are molar fractions in the repeat unit, satisfying x < 0.8 and x + y + z = 1, except for the case where x, y or z is 0. <EOS>