Based on the data recorded by 67 stations in the Gansu digital seismic network and its adjacent seismic network from 2000 to 2015, we relocated 4 592 small earthquakes that occurred at the epicenter of the 1927 Gulang M8 earthquake and in its adjacent areas via a double-difference earthquake relocation algorithm. The results show that the eastern and western segments of the Huangcheng-Shuangta fault zone exhibit different mechanical motion properties. While the western segment is primarily dominated by thrust faulting, and earthquakes occurred mainly in the footwall of the fault, and in the eastern segment earthquakes mainly occurred in the hanging wall of the fault. Moreover, we first found a conjugate fault located in the middle segment of the Huangcheng-Shuangta fault that nearly perpendicular to the main fault. Based on the inversion of fault parameters of these small earthquakes, we also found that this high dip-angle fault mainly exhibits right-lateral strike-slip faulting with a minor tension component.