The Sanweishan fault, an important branch of the eastern segment of the Altyn Tagh fault and located in the leading edge of the northwestern Tibet Plateau is a left- lateral strikeslip and thrusting activity. Field investigation and satellite image interpretation reveals that the main fault, after crossing through Mingsha Mountain, continues to extend westward and finally crops out in Danghe water reservoir segment (DHRS) and Yangguan County in the form of fault escarpment. The length of DHRS is almost 16 km, strikes near 90°. Morphology of the fault escarpment changes from simple to complicated with the fault crossing Yangguan County westward to DHRS and from Mingsha Mountain to Sanwei Mountain. The difference of the escarpment ranges from 2 to 6 m, but it tends to decrease and horizontal displacement also reduces. All of these phenomena reflect that the intensity of fault activity weakens gradually from west to east. The results of trenching and section interpretation show that the fault offsets the Lower Pleistocene sandstone and conglomerate (Yumen Formation, Qp1), the Middle Pleistocene gravel layers (Jiuquan Formation) and the Upper Pleistocene alluvial gravels. Satellite image interpretation, field investigation and magnetotelluric profiles indicate that the Nanjieshan fault intersects with the Sanweishan fault on the south side of the Mogao Grottoes, and the two faults merge to the Altyn Tagh fault at depth. The above three faults constitute an asymmetric positive half- flower structure with regional compression. The extension of the northern margin of the plateau may result from the activities of an array of NE- SW strike- slip faults and the northeast movement of the blocks between those faults.