Abstract: Logic delayed circuit is an important method of discriminating radium isotopes by indirectly measuring radon isotopes. Based on the theorem of delayed-coincidence system，a method of software delay coincidence discriminating radon isotopes were proposed. The uncertainties of different delay time and gate time were studied. The results showed the optimum parameters. The software method is a alternative to hardware, it can significantly reduce the cost and also offers convenience and flexibility. At the same time, this method can be used in radon isotope measurement of low activity.