After the 8.0 magnitude earthquake that occurred in Wenchuan County, Sichuan Province，the authors established monitoring points of aftershock in Ningqiang County, Shaanxi Province. The relationship between the aftershocks and the Rn-Hg content change in soil gas was observed on the spot and at regular intervals in 17 days. The results show that the soil gas mercury content is generally more than 200 ng/m3 in the hanging wall of the earthquake fault， the content value is two times higher than that of the footwall（In fractures along the section，mercury anomaly value can reach 560 ng/m3），but radon content value shows no distinct change. Along the strike of the seismogenic main fault, the closer it is to the epicenter, the higher the Rn, Hg background values are. If mercury content is more than 20 ng/m3 in the background section, there is Rn anomaly simultaneously, which Indicates that there might occur higher probability of aftershock with highfrequency and over 4.0 magnitude within 300 km and 4～48 hours; if things are not like that, the aftershock will not occur.