A SN-trending left-slip thrusting active fault has been found at the western Yinwashan margin in northern part of Jiuxi basin, western segment of Hexi corridor. The fault is about 4km length, extends SN-trending and dips to east at a middle angle. As a second-order active fault, it developed near the northwestern end of the NW-trending Yinwashan light-slip thrusting active fault. Along the fault, the greyish–green Ordovician epimetamorphic fragmental rock has been trusted upon the Cretaceous purplish red sandstone and conglomerate, Neogene tangerine mudstone and even upon the loose slop wash (TL dating is 10.3ka) of last stage of late Pleistocene. The fault shows a large obduction movement from the east side to west side in section and the last neo-structural activity occurred at the middle-late Holocene (about 3～5ka BP). It also offsets EW-trending gully from east Yinwashan Hill to west clinoform low hills or Gobi Desert and results in the gully curving as S-type in plane, which indicated the fault has an evident left-slip movement. The diagonal displacement along the fault plane is about 3000m and the vertical displacement is about 1800m, the later equal approximately to the vertical displacement of Yinwashan active fault or the uplifted height of Yinwashan Hill. The fault is result from the partial tectonic stress field caused by light-slip thrusting movement of the Yinwashan active fault. On the other hand, the large displacement of the Yinwashan active fault is balanced in a short distance by the left-slip thrusting movement of western Yinwashan SN-trending active fault.