Abstract: The delay coincidence measurement of ²²⁰Rn and ²¹⁹Rn has been proved to be a valid indirect method for measuring ²²⁴Ra and ²²³Ra extracted from natural water, which can provide valuable information on estuarine/ocean mixing, submarine groundwater discharge, and water/soil interactions. In practical operation chance coincidence correction must be considered, mostly Moore’s correction method, but Moore’s and Giffin’s methods were incomplete in some ways. In this paper the modification (method 1) and a new chance coincidence correction formula (method 2) were provided. Experiments results are presented to demonstrate the conclusions. The results show that precision is improved while counting rate is less than 70 min^-1.