Geochemical survey is an effective and important method in prospecting for geothermal systems. As geothermal reservoirs are closely related to structu res, it is essential for us to. investigate the structures contiolling the geothermal water in the process of geothermal field exploration.This paper summarizes the geochemical methods available for detecting faults in the geothermal system, with special attention paid to some testing results of gas geochemical survey performed in the last few years for faults in geothermal fields. It is shown that such methods as mercury vapor in soil, ²¹⁰Po, Alpha-card and in-situ goil-gas CO₂ accumulative determination are all effective mneas for detecting faults in the geotheraml area. Our study has also proved that the multi-parameter survey is much more effective and reliable than the single-parameter survey. The geochemical survey can be used not only to find faults but also to disclose concealed structures in the geothermal system.