Abstract: Based on the response of well water level to tide st ress, the autho rs analyzed the response of well-aquifer to st ress v ariation during the process of earthquake preparation. While the instantaneous st ress variation of aquifer is more than the critical value, the chang epat tern of water level anomaly is leap. If the instantaneous st ress variation of aquifer is less than the critical value, but accumulation of stress variation of aquifer is more than the critical value in the course of time, water level anomaly is g entle. How ever, decline or rise of water level anomaly depends on the tectonic position where the well aquifer lies in the process of stress regulation and re-accumulation. Tha tis tosay, the feature of water level anomaly before strong earthquake is resulted f rom the regio n where stress anomaly was fo rmed during the process of earthquake preparation.

Key words: Well water level, Stress anomaly region, Anomalous mechanism