Abstract: Based on the re-locating earthquake catalogue and more additional data,the focal mechanism solutions of the Tangshan earthquakes of Ms6.0 are computed again. Their rupture geometries and source parameters are also re-estimated by means of the new and old methods respectively. The aftershock seismicity in the period since the mainshock of 1976 to the end of 1984 is studied in some detail.The new results show that the focal mechanism solutions of the earthquakes of Ms=7.8 and Ms=7.1 become more similar to each other and their rupture directions are almost the same. The rupture length of the Ms = 7.8 earthquake is estimated as 77-87 km,and its northeastern end terminated in the region near Zhenzizhen,instead of extending to the region of I.ulong. According to the new results,a new mechanical model of echelon fractures with unequal lengths is presented in this paper. The theoretical and experimental results indicate that the echelon fracture model can simulate the fracture processes of the Tangshan earthquake better than other models. By using this model,more observed facts that could not be explained with other models,can be explained more satisfactorily.