Abstract: Before strong earthquakes, certain anomalous features may show up in the seismicity of a broad region possibly caused by the concentration of stress and the ensuing anelastic deformation at the source region of the earthquakes. Certain parameters of seismicity with rather obvious physical meanings and supported by the results of rock fracture experiments, are selected for the study, namely the areal extent A of the seismic active area, seismic energy released E, accumulative numbers N of earthquakes and the value b of large and small earthquakes and so on. A method of scanning has been adopted to search at first for the dangerous location under which a strong earthquake is in the process of development and then by applying time scanning to show further the transition from medium to medium-short range of anomalous features of seismic activity before the occurrence of a large earthquake.On the basis of scanning on the observed seismicity data of North China, Southwest China and Northeast China before several large earthquakes, emphasis has been laid on the anomalous seismic feature of North China before the 1976 Tangshan Earthquake of magnitude 7.8. The paper also discusses the different roles played by the parameters of seismicity in mediumterm prediction of strong earthquakes. It is believed that such a method of time and) space scanning, particularly of the b value has certain significance in medium-term earthquake prediction.