In this paper, we have discussed the principal characteristics of the regional seismicity before the Tangshan earthquake(M_s=7.8) in particular during the mid-range of the development of the strong earthquake. After 1975 year, the anomalous variations undergo transition and recovery. These phenomena and low stress drop of main shock may show that during a period before the strong earthquake, the stress decreased with the further development of the strain.Analysising the anomalous increasing of ground-temperature near the seismic centre since 1975 year, it is suggested it is possible that as hot matter gush up from the deep of the earth,the strength of the medium decrease, so that the strong earthquake takes place.