Abstract: According to the strata parameters such as characteristics of three-dimensional seismic reflection, variation in thickness of sedimentary formation, abrupt capacity of oil well, and differences of formation pressure coefficient and dip angle, the existence and the property of No. Ⅻ fault in the west part of Qaidam Basin were investigated by analyzing the structural evolution. The result showed that No. Ⅻ fault existed between Yuejinyihao and Shaxi structures in the west part of Qaidam Basin. This fault was a normal fault resulted from extension in the early stage and then turned into reverse fault under extrusion. The fault was the oil-gas migration pathway when it was open. The independent oil-water system formed in Yuejinyihao and Shaxi structures when the fault was sealed. Because of existence of this fault, necessary steps should be taken to stabilize oil well yield and improve well deliverability during the development of Yuejinyihao and Shaxi oilfields.