Abstract: A earthquake series is referred as the events being successively occurred in a definite time and in the same seismic area. These events are characterized by the common developing process and an unified evolutional history of precursory field. We consider that the seismic precursory field is nothing but a regional field beyond a certain source region.For the North China fault block region can be recognized two earthquake series, Xingtai-Bohai and Haicheng-Tangshan series ever since the occurrence of Xingtai earthquake of 1966 to the year of 1978. The anomalies in the precursory field of this series started in 1972, after 1976 Tangshan earthquake it has been gradually becoming quiet. With regard to the developmental process of this field and its spatial distribution can be distinguished following five anomaly-development periods and relevant six stages.1. The primary period (A), during which the precursory field is developed from the stage in a scattered form (Ⅰ) to a divergent stage on a large scale (Ⅱ).2. Accelerated period (B) before Haicheng earthquake, during which the precursory field migrates northward entering into a convergent stage (Ⅲ) in the extent.3. Developing period (C), during which the precursory field enters into a rediver-gent stage (Ⅳ).4. Transitional or accelerated period before the Tangshan earthquake (D), during which the precursory field enters into a reconvergent stage(Ⅴ).5. Impending period before the Tangshan earthquake (E), during which the precursory field enters into a divergent stage (Ⅴ).It is suggested that the formation and development of the precursory field are closely related to the conjugated shear-rupturing network, and the distribution and development of anomalous points or zones are constrained by slipping of conjugated shear-ruptures and stress concentration at the various tectonic position. Consequently, the precursory phenomena would not be limited in the source area and its neighbouring regions, and the multi-anomalous regions (zone) are formed.Finally, the relationship of the regional precursory field with the earthquake prediction is studied. In general the regional precursory field does not imply only one event, perhaps an earthquake series, and the precursory extent has a certain relation to the magnitude of an event. To discriminate the location of one shock from the earthquake series it is necessary to pay attention to the spatial changes in the precursory field, such as the migration and convergence of anomalies. On the background of regional precursory field, the macroscopic anomalies in a great amount and concentrated form together with some abrupt anomalies can be taken as a precursory signal of a big imminent earthquake.