The invention discloses a method for improving the carbon dioxide adsorption performance of activated carbon. The molecular sieve similar type activated carbon is improved by an impregnation technology, and impregnation liquid comprises a 1-3 mol/L potassium sulfate solution and a 0.5 mol/L ammonium persulfate solution; the method comprises the steps of fully mixing the impregnation liquid and the activated carbon until no bubble is generated, and heating up to 50-70 DEG C; carrying out heat preservation until the solution has complete reaction and disappears; washing the activated carbon by distilled water; and after that, drying the product to constant weight at the temperature of 120-150 DEG C to obtain the activated carbon product. The adsorbing capacity for carbon dioxide of the prepared activated carbon product is increased by 20-40% based on the original adsorbing capacity for the carbon dioxide. A device and a method for testing the adsorbing capacity for the carbon dioxide of the activated carbon are shown by figures. The activated carbon product prepared by the method has the characteristic of being good in gas phase absorption effect, is used for a purification technology of high-purity gas, and is capable of greatly improving the working efficiency.