Radon measurement, which is used to demarcate the fault (or fracture) and its opening degree, and to evaluate its hy - drogeological characteristics, is an effective and quick way. RnA (daughter of Rn) concentrations were measured at three test sec - tions in Xiaoyudong active fault. On the basis of a statistical analysis of the acquired radiometric data, background value and ab - normal value of Rn gas were determined. The active fault was classified into different openings, and its hydrogeological meaning was discussed. This work can be used to delineate the location of groundwater resources in fractured-rock aquifers and to select the borehole sites for water supply purposes.