The parameters of mercuric gas in water at the Qicun area in Shanxi Province are mainly controlled by the regional hydrochemical characteristics of Qiding basin;they have sensitive precursory effect to earthquakes especially to that with larger than 4.0 magnitude in Shanxi Province.The abnormal effects of mercuric gas in water are short and temporary;But,it can provide useful information of predicting the time of earthquake,which is one of the three elements of earthquake(time,location,and scale).The abnormal effects of mercuric gas in water are also concerned with the activity level of earthquake as well as depth of earthquake and so on.