A fixed-bed apparatus with the dynamic two-route proportional gas mixing system was designed to investigate the adsorption behavior of carbon dioxide on 5A molecular sieve for designing the helium purification system of high-temperature gas-cooled reactors (HTGR). Experiments were performed at near atmospheric pressure, and the dynamic column breakthrough method was used to derive the single-component adsorption equilibrium of carbon dioxide with helium as the carrier gas. The breakthrough curves and the desorption curves were measured at low carbon dioxide partial pressure of 10-150 Pa and temperatures of 5-35 degrees C. By analyzing the breakthrough curve, both the equilibrium adsorption capacity and the kinetic adsorption capacity at the breakthrough point were determined. The effects of superficial velocity, temperature, gas feed concentration, adsorption pressure, and regeneration temperature on the adsorption capability were studied. The CO2 single-component adsorption isotherm at 10 degrees C was obtained and fitted well by the Langmuir and the Freundlich isotherm models. The heat of adsorption for carbon dioxide on 5A molecular sieve was estimated.