This work applies the double- difference tomography method for the tomography and relocation of earthquakes in the source area of Luding MS6.8 earthquake and its surrounding areas using the seismic phase data of 103 990 earthquakes (27.5°-31.5°N, 100°-104°E) recorded by Sichuan digital seismic network. Results show that: (1) the aftershock sequence is mainly distributed on the southwest side of the Xianshuihe fault in the NW direction, with a length of about 60 km. The focal depths are mostly concentrated at 5-15 km and exhibit symptoms of increasingly deepening from north to south. On the basis of the spatial distribution of the aftershock sequence and the location of the mainshock, the southeast section of the Xianshuihe fault is proposed as the seismogenic fault. In addition, branch faults may have occurred at the northern and southern ends of the aftershock sequence. (2) On the basis of the spatial distribution of background earthquakes, foreshocks of the Luding mainshock are speculated to have occurred. The high- velocity anomaly body on the northwest side of the mainshock may block the NW- trending rupture of the Luding earthquake. Given the weak mechanical property of the southeast section of the Xianshuihe fault, the Luding earthquake is inferred to have ruptured unidirectionally in the SE direction. (3) A weak S- wave low- velocity layer and a high- wave velocity ratio are found at a depth of 30 km below the focal area of the Luding earthquake. Previous studies found a high- conductivity layer and a low- resistivity layer in the mid- lower crust of the study area. The mainshock is located near the earth heat flow isoline with a heat flow value of 65 mW·m², so fluid may be found in the crust at 30 km below the focal area. Furthermore, the Luding earthquake occurred at the junction of high and low velocities where stress is easy to accumulate and release, resulting in rupture. A large amount of stress might have accumulated in the focal area, and the fluids in the crust flow into the seismogenic fault, weakening the fault. Finally, the combined effect of the two factors led to the occurrence of the Luding MS6.8 earthquake.