The invention relates to an erythrocyte life span determining method and device. The method is a non-dispersive infrared spectroscopy method, cleaning gas stable in performance is used for cleaning a gas chamber, the problem that needed samples are large in volume is solved by adopting the method of intermittent and repeated sample feeding in a small amount, an interference element absorbing bag is used for removing interference elements, an alveolar air sample and background air sample paired measurement and paired level difference-concentration difference value fitting method is adopted to obtain alveolar air endogenous CO concentration, and the CO gas chamber and CO2 gas chamber method is adopted to measure CO2 concentration and CO concentration through the same machine; according to the measured CO2 concentration, the influence on alveolar air endogenous CO concentration by entrained air when alveolar air is collected is corrected, the accurate value of alveolar air endogenous CO concentration is obtained, and the erythrocyte life span is calculated. The device is an instrument which is designed according to the method and is composed of a gas chamber unit, a transmission unit, a circuit unit, a power source unit, a gas path system connected with all the units by electrical connection.