Abstract: Before Zhangbei earthquake, in mid term stage, water radon in many wells showed negative anomalous variation;in short term stage, the monthly frequency of the differential anomalies of regional radon increased;in imminent stage, water mercury in Huailai No. 4 well showed many sudden jumps. Before the Datong earthquake, water radon in many wells showed trend positive anomalies, which is obviously different with the hydrochemical precursors of Zhangbei earthquake. In this paper, the reason of prediction failure by hydrochemical method to Zhangbei earthquake is analyzed. It is considered that the confidence of water radon negative anomalies which are regarded as mid term precursors is low. In order to raise the effectiveness of earthquake prediction by hydrochemical method, the method must be combined with other precursory methods.