On April 20, 2013, an earthquake with a magnitude of Ms7.0 shocked the region of Lushan County, Ya'an City, west Sichuan. The epicenter is located in the zone between Shuangshi and Taiping Townships, with a focal depth of about 13~14 km. The maximum intensity degree attained IX. Although severe destruction of buildings in the shocked area, field investigation did not find obvious co seismic surface rupture, except a small quantity of grand fissures with sand liquidation. Integrate analysis of high resolution satellite images, main and after shock distribution and focal mechanism shows that this earthquake is an independent reverse faulting event possibly rupturing the southern segment of the Shuangshi Dachuan fault, along the SW segment of the Longmenshan fault zone. We infer that the main shock may have occurred on a west dipping ramp (with dipping angle of 33°~43°) of a sole décollement beneath the SW Longmenshan, a similar seismotectonic setting of the Wenchuan earthquake. Historically, there recorded at least two earthquakes with magnitude of Ms 6~6.5 along the Shuangshi Dachuan fault, together with the Lushan earthquake, they characterize the fault activity behavior of this fault zone. In situ stress measurement results show that this earthquake resulted from the brutal release of accumulated crustal stress within the SW segment of the Longmenshan fault zone.