The invention discloses a system for separating and purifying inert gases in a nuclear power plant. The system comprises a gas delivering unit, a gas adsorbing and filtering unit, an iodine isotope retention unit, an activated carbon adsorbing and desorbing unit and a chromatographic separation and purification unit, wherein the gas delivering unit is connected with gas storage equipment or an exhaust opening of the nuclear power plant and is used for providing an air source; the gas adsorbing and filtering unit is connected with the gas delivering unit and comprises a gas washing basin, a modular sieve drying unit and a filtering unit which are sequentially connected; the iodine isotope retention unit is provided with a first activated carbon cold trap; the activated carbon adsorbing and desorbing unit is in pipeline connection with an outlet of the first activated carbon cold trap and comprises at least one group of activated carbon adsorption components; and the chromatographic separation and purification unit is in pipeline communication with an outlet of the activated carbon adsorbing and desorbing unit and comprises a chromatographic instrument provided with a chromatographic column, a sample injection control system and a detection device. Compared with the prior art, the device disclosed by the invention has the advantages of simplicity, convenience in operation and capability of preparing high-purity inert gas with a single element. Besides, the invention also provides a method for separating and purifying the inert gases in the nuclear power plant.