Abstract: Based on the distribution of H₂, Rn and CO₂ concentrations in the Tangdong active fault zone determined by multi-phase cross-fault observation in the field, this paper analyzed the gas geochemical characteristics and tectonic activity background of the fault zone, and revealed the relationship between geochemical characteristics of soil gases and the geological structure. The results showed that there were significant variations in H₂, Rn and CO₂ concentrations in different measurement periods. The gas concentrations of H₂, Rn and CO₂ in June were significantly higher than those in other measurement periods on the Zhanghecun measurement profile, while the gas concentrations of H₂, Rn and CO₂ in January were significantly higher than those in other measurement periods on the Xinglizhuang measurement profile. And the background values of H₂ and Rn on Zhanghecun measurement line were (8.93±3.92）×10⁻⁶ and （17.38±4.28） kBq/m³, respectively, and concentrations of H₂ and Rn exhibited synchronous anomalies at 135 m and 230 m from the main fault of Tangdong in the west of the measurement line. Accordingly on the Xinglizhuang measurement line, background concentrations values of H₂ and Rn were （41.20±16.64）×10⁻⁶, （29.00±8.28） kBq/m³, and concentrations of H₂ and Rn showed synchronous anomalies at 60 m from the main fault of Tangdong in the west of the measurement line. Furthermore, the shape of the gas concentration curves were similar for different measurement periods, and the anomaly spike showed perfect reproducibility on each investigation profile. There was a good correspondence between the high-value anomalies of the two investigation profiles and the results from geophysical exploration and trans-fault joint drilling. Therefore, it is deduced that the concentrations of H₂ and Rn, which could be sensi-tive to the fault location, are the key indicators for gas geochemical observation of Tangdong fault zone.