The invention discloses a method for rapidly measuring radon concentration by zero-order approximation, comprising measurement and calculation. At first, a radon measurement instrument is enabled to work under a scale mode, radon concentration is reversely deduced by using the counting of a long-time 218Po peak in a standardized radon chamber to obtain a scale factor, which is therefore equivalent to an RAD7 radon measurement instrument working under a sniff mode; the radon measurement instrument is then switched to work under a zero-order approximation mode, measurement cycle and cycle times are set, and after the radon measurement instrument is started up, the air outside is subjected to the filtration of radon daughter element by a high-efficiency filter, then continuously pumped into a measurement chamber and discharged to the outside; new daughter element 218Po generated by radon decay in the measurement chamber is adsorbed to the surface of a semiconductor detector under the action of strong electric field, electric signals, which are generated by high energy alpha particles released by the further decay of the 218Po, is subjected to energy identification and then counted to obtain the concentration of the 218Po so as to obtain radon concentration; and the data of radon concentration obtained by measurement is then calculated by applying a specific algorithm, thus the change of radon concentration can be rapidly measured.