Disclosed is a method for protecting terminated amine in a polybenzimidazole molecule by carbon dioxide, which is characterized in that polybenzimidazole is dissolved by a solvent, under the protection of nitrogen and action of catalyst, CO2 gas is input for reaction to generate the polybenzimidazole of which the terminated amine is protected, wherein, the solvent adopts dimethyl acetamide, dimethyl sulfoxide or N-methyl-pyrrolidone, and the catalyst adopts concentrated sulfuric acid, phosphoric acid, pyrophosphoric acid, polyphosphoric acid, solid superacid or a molecular sieve. The method is more stable than that using a carbonyl compound to generate Schiff alkali or using an acyl compound to generate amide; and is easier to perform than that using benzoic acid to protect reaction, mass or excessive carbon dioxide can be added, and excessive carbon dioxide can be removed easily.