In order to determine the geochemical characteristics of soil gas in the central south segment of Xiadian fault and evaluate its seismic hazard, the authors measured the concentrations of He, H₂, CO₂ and Rn in soil gas at 221 measuring points distributed in Qixinzhuang, Pangezhuang and Dadongguan sites across the fault. The geochemical characteristics, the background values and the anomaly thresholds of He, H₂, CO₂ and Rn in each sampling profile were identified. The data obtained indicate that various soil gas anomalies occur along the fault, and the utilization of various components of soil gas can be an effective way to identify the concealed fault. In addition, based on the anomaly amplitudes of He and Rn along each sampling profile, the authors infer that the activity of Xiadian fault in Pangezhuang is stronger than that in Qixinzhuang and Dadongguan. However, the decrease of the measured peak hydrogen concentrations in Xiadian fault and the seismic section data reveal that the activity of the central south segment of the fault was somewhat weakened.