Abstract: The method for measuring ²²⁰Rn progeny aerosol activity particle size distributions was introduced through ELPI system, α spectroscopy and the energy discrimination method. The different particle sizes of the ²²⁰Rn progeny aerosols were collected and the activity size distributions in the ²²⁰Rnlaboratory of the University of South China were measured by this method. The experiment results show that the activity median aerodynamic diameter (AMAD) of ThB aerosol is 237 nm, and that of ThC is 245 nm. The simple and quick method can be used to monitor the particle size distributions of ²²⁰Rn progeny aerosol in real time, the aerosol activity size distributions of ThB and ThC can be obtained by this method at the same time, and the measurement accuracy of the energy spectrum is higher than that of custom method.