Abstract: A M5.1 earthquake occurred at Wen'an of Hebei Province on July 4 of 2006.The water mercury from Zi-512 well in Macun Station of Shijiazhuang, 200km southwest from the epicenter, showed a high value anomaly from March 15 to May 15 and a low value anomaly on June 3 before the event.The characteristic of the anomalies is studied here.As its large magnitude and clear characteristic, the anomalies have great significance of precursor.