This paper introduces the measuring method of gas mercury content in soil, the selection of optimal observing conditions of JM-3 gold membrane measuring instrument and the observing results of gas mercury content in soil in A'erjin Fault and its vicinity. Before the Menyuan earthquake(M=6.4, 1988)and the Lancang earthquake of 1988 etc., the sudden changes of gas mercury concentration were observed. The Xiwudang earthquake, Zhangye, Gansu Province on Sept. 12, 1987 was predicted using the change of gas mercury concentration in soil.