Abstract: The study on accurate measurement of ²²⁰Rn activity concentration was carried out theoretically and experimentally. The effect of the volume of dryer vessel and the sampling flow rate on ²²⁰Rn measurement was studied in detail. It is found that the increase of dryer volume and the decrease of sampling flow rate can increase the differences between RAD7 results of ²²⁰Rn activity concentration and true value. For an accurate measurement of ²²⁰Rn activity concentration, the correction is necessary. And the results show that correction factors for both the RAD7 proper small dryer and the big dryer are 1.224±0.118 and 2.232±0.598, respectively.