The preferred area for high- level radioactive waste (HLW) geological disposal repository in China is located in the southern margin of the Beishan area, Gansu Province. There are many large- scale non- active faults in the area. These faults likely provide the dominant pathways for groundwater (GW) flow. In the process of extending to the surface,some key parts are hidden underground and difficult to identify due to Quaternary coverage and other factors. In order to identify the hidden parts in large- scale faults and its hydrogeological significance, experimental sites were selected located in the south of Xinchang site in the Beishan area, namely the Hongqishan and the Qianhongquan faults. The soil radon concentrations were obtained in both these faults. The results suggest that measuring radon concentrations in surface soil is useful for identifying the hidden faults in this region, and is an important supplement to the traditional geological survey method. The opening of the fracture networks of the two fault zones is undeveloped, and the damage zone is small. There is a high possibility of forming large- scale GW storage space. The background values of soil radon concentration in different fault zones are quite different, which could be due to the cumulative frequency distribution of soil radon concentration. The contour distribution map of soil radon concentration can indicate the location of hidden faults. For the compression shear faults, the main faults are mostly distributed along the boundary between high and low radon anomalies or show medium anomalies, while the secondary faults on both sides show high anomalies.