Abstract: In this paper,the geological structure of Huailai No.4 well has been introduced first,then the relationship between the mercury anomalies and earthquakes before 4 moderately strong earthquakes has been analyzed according to the mercury observtion data in Huailai No.4 well from 1989 1998.The result shows that the mercury anomalies showed a relatively good corresponding relation to the seismic activity anomalies before earthquakes.