Abstract: Based on the consideration of combining "source, field, outside", the anomaly mechanism caused by groundwater in shallow layer is put forward.The dynamic variation of ground water caused by rain may produce an additional force to the rock.Before an earthquake, since the action of many fluid forces, the crustal stress field begins to adjust, and change the intensity of rock so that it causes tectonic movement of shallow crust.The result is many precursory anomalies in shallow layer associate, such as crustal deformation, crustal stress, hydrochemistry and earth resistivity.Among many periodical variation of rain and groundwater, only those that can be coupled with stress strain course can cause anomaly, which can be defined as generalized precursor.The principles of pore pressure, fluid pressure and chemical erosion are the theoretic foundation of this anomaly mechanism.As direct and indirect evidence of the mechanism 6 facts are presented.