Obiective:On the north side of the Hexi Corridor, southern edge of the Beishan area, an active fault was discovered and we named it as Beishan southern margin fault. However, the movement pattern and activity age of this fault are unknown. Meanwhile, the special location of this fault may be important for understanding and identifying the northern growth pattern of Qinghai—Xizang (Tibet) Plateau. Methods: Based on interpretation of satellite images, investigation of fault geomorphology, trench and OSL dating method, this study gave the activity characteristics of this fault. Results: A series of ruptured landforms were formed because of the fault activities since the Late Quaternary, including synchronous elbow displacement of gullies that cross the fault due to the left- lateral strike- slip movement of fault, the amount of displacement is 4.5~6.2 m, and the average value is 5.2 m. The northward thrust of the fault forms a clear linear fault scarp with height of 0.2~1.4 m. In addition, four trenches along the fault revealed at least two sudden quick displacement events, the older one occurred in 30 ka ago, and the newer one occurred between 8.4 ka and 3.3 ka. Conclusions: Beishan southern margin fault is located on the north side of Jinta County, about 50 km from the county seat. The length of the fault is about 18 km. Strike of fault is near east—west direction, dip is northwest, and the dip angle is 60°~80°. Combined with the evidence from satellite imagery, geomorphology and trenches, it is believed that the fault has been active in the Late Pleistocene and Holocene, and is an active fault with left- lateral strike- slip and thrust property. The activities of this fault may reveal that the extension of the northeastern margin of the Qinghai—Xizang (Tibet) Plateau has crossed the Hexi Corridor and entered the Beishan area.