Abstract: Using the recorded waveform from the Regional Seismic Networks, the focal mechanism solution of the Tangshan M5.1 earthquake that occurred on July 12, 2020, in Hebei Province was investigated by the gCAP method. The strike, dip, and slip of the first nodal plane are 150°, 76°, and 14°, respectively, those of the second nodal plane are 243°, 76°, and -166°, and the moment magnitude is M_W 5.01. The results show that the centroid position is 39.668°N, 118.295°E, and the depth is 18 km using the grid search method in the 3D vicinity of the hypocenter. The result's robustness was investigated by determining the center of several focal mechanisms of the same earthquake and their standard deviations. The resolved focal mechanism is stable. Preliminary results show the second nodal plane is inferred to be the fault plane by hypocenter-centroid methods for quick identification of an earthquake fault plane. For validating the results, combining the characteristic of geologic structure in the seismic source area, and the space distribution of aftershock sequence and isoseismal line of intensity, we conclude that the second nodal plane is inferred to be the seismogenic fault plane, and the Tangshan-Guye fault is the seismogenic structure. The mainshock source mechanism exhibits that its main characteristic is a dextrorotation strike-slip mechanism. We deduced that the earthquake may indicate the process of stress release controlled by deep and shallow structures, which is a powerful seismic activity of the Tangshan fault zone.