Abstract: This paper introduced a measurement method of low concentration ²²⁰Rn—time interval analysis (TIA) method. In the short time after the alpha particle emitted at the decay of a ²²⁰Rn atom an alpha particle from the decay of ²¹⁶Po (half life 0.145) follows. The multi-time analysis method is based on the analysis of the emitting time interval between the two successive alpha particles, which can be used to discriminate the ²²⁰Rn from ²²²Rn. A flow through Lucas cell (FD125) was used to measure 10 Bq standard ²²⁰Rn source. When the ²²²Rn activity is less than 10 times the ²²⁰Rn activity, count rate is controlled within 60 min^-1, the time resolution is set to 1 ms and the measuring time is 10 h, and the relative deviation of the results is within 7%.