Abstract: Basing upon the data obtained from long-term observations of ground water regime and taking the Tangshan earthquake as an example, the authors proposed first the principles and approaches to recognize the precursory anomalies of earthquakes and classified the subsurface water regime into underground water, interlayer water and confined water. For the Tangshan earthquake the forerunner field of underground water is characterized by its spatial distribution (limited to a certain quadrant), its stages of development in time and the synchronism of anomaly-changes during and after a shock.Furthermore, this study stresses the importance of these principles mentioned above in predicting the space, time and magnitude of future earthquakes and discusses the relationship between the focal stress field and regional stress field and the indicator to distinguish them. It is suggested that the former develops progressively and its principal axis of compression stress changes in direction just before an earthquake, thus enabling us to divide the focal process into two basically different stages-brewing and originating stages.