Abstract: The software delayed coincidence counting technology was researched based on embedded system. The device based on ARM processor was constructed to record time serials information of nuclear pulse. According to the different half-lifes of Po short-life daughters of radon isotopes, a method of software delayed coincidence counting was used to discriminate radon isotopes. Experiment results of ²²⁰Rn and ²¹⁹Rn standard source measurement show that the software delayed coincidence counting method is practical for discrimination measurement of low level activity of radon isotopes. The software method is an alternative to hardware one, and it can offer better convenience and flexibility.