On the basis of analysis of observation data for mercury content in Songshan spring of Yanqing county, Beijing city, reflecting earthquake ability of mercury anomaly of the spring is studied and evaluated.The results show that there is a better correspondence between mercury anomalies of the spring and earthquakes. Especially, there is obvious correspondence between the anomalies and mid small earthquakes near the spring and M_S ≥5 earthquakes far away the spring since 1997. The cause of sensitivity reflecting earthquake for the anomalies of the spring is discussed from activity of geological structures around the spring.