Abstract: This paper deals with the determination of seismic parameters of the Tangshan earthquake and its geological background, and also discusses characteristics of microshocks before the main shock, such as seismic gap, shock-concentration-quiescence, b-value decrease and the strip-distribution of shocks and so on.According to the surface rupture trend caused by this earthquake, the initial motion pattern of P-waves, the vertical deformation both before and after main shock and the distribution of aftershocks etc. we supposed that the Ms 7.8 earthquake probably was a result of the horizontal dextral movement of NE trending fault, and the Ms 7.1 earthquake was caused by the horizontal sinistral displacement of nearly SN running Luan-he fault which was dragged by the NE fault.From the characteristics of space and time distribution of aftershocks in Tangshan area, the focal rupture process and its influence on the aftershocks were discussed in this paper. Finally, it is suggested that the occurrence time of Tangshan earthquake was probably associated with the current astronomic conditions.