The invention relates to a measuring method and system for the diffusing capacity of CO in multi-component gas. The measuring method for the diffusing capacity of the CO in the multi-component gas comprises the following steps of determining ambient atmospheric pressure, ambient water vapor pressure and an inspired gas volume of a human body; irradiating standard gas with a first light beam of detection equipment, and detecting a light intensity change of the first light beam to obtain a first CO concentration and a concentration of CH4 in the standard gas; irradiating the standard gas with asecond light beam of the detection equipment, and detecting a light intensity change of the second light beam to obtain a second CO concentration; irradiating expiratory gas with the first light beam,and detecting a light intensity change of the first light beam to obtain a third CO concentration and a concentration of the CH4 in the expiratory gas; irradiating the expiratory gas with a second light beam, and detecting a light intensity change of the second light beam to obtain a fourth CO concentration; and performing calculation according to a formula to obtain the diffusing capacity of theCO in the expiratory gas. The precision of the diffusing capacity, measured by the measuring method, of the CO is higher.