Abstract: The variation of water radon anomalies before Zhangbei M_s6.2 earthquake (1998)has been introduced.Based on the correspondence of water radon anomalies with the medium and large earthquake in the northwest region of Beijing,the character of water radon precursory anomalies in Huailai No.2 well has been summarized.The anomalies'shape character is from decreasing to increasing.The anomalies'functioning range is about 120~160km.Earthquakes usually occur within 28~55 days when the anomaly appear.