At 2:04 am on May 22, 2021, a MS 7. 4 earthquake occurred in Maduo County, Guoluo Tibetan Autonomous Prefecture, Qinghai Province. Its epicenter is located at 34. 59°N~98. 34°E, ~70 km south of the East Kunlun fault on the northern boundary of the Bayan Har block, with the focal depth is about of 17 km. Detailed field investigation from the days right after the earthquake showed coseismic surface ruptures consisting of a series of tension cracks, tension shear cracks, shear cracks, push- ups, and collapses at an altitude of 4200~4600 m. The ~N105°E- striking surface rupture zone is ~151 km- long and is characterized by left- lateral strike- slip movement, with minor normal component at places. It follows the Jiangcuo fault located just south of the East Kunlun fault. We divide the surface rupture zone into four segments, according to the geometric changes in strike direction and structural steps/bends between the segments: western, central western, central eastern, and eastern segments. The western segment consists of two branches: the northern splay is discontinuous along ~18 km and strikes N112°E, while the southern splay is continuous for ~25 km and strikes N94°E. It is along the southern splay that we found the largest coseismic left- lateral offset: ~2. 9 m. The central western segment is ~52 km- long, with continuous surface ruptures in the westernmost ~22 km (striking N109°E) and ~30 km of discontinuous rupture in the east. The largest left- lateral offset of ~1. 9 m can be found along that central western segment. The surface rupture along the central eastern segment strikes ~N104°E discontinuously for ~51 km, including a 20 km- long gap without any surface rupture. That along the eastern segment consists of three branches: the northern rupture strikes ~N84°E continuously for ~23 km, with the largest left- lateral offset of ~1. 8 m. The central rupture strikes ~N110°E discontinuously for ~14 km. The southern splay only has minor ruptures and collapses striking ~N120°E for ~6 km. The rupture pattern of large- scale branching occurred at both ends of the surface rupture zone and has never been reported along other coseismic surface rupture zones of strike- slip earthquakes on the Tibetan Plateau. The seismogenic fault of this earthquake is the Jiangcuo fault, which may connect with the Kunlun Pass surface rupture zone of the 2001 East Kunlun (Kokoxili) earthquake (MS 8. 1) to the west, suggesting that the Kunlun Pass- Jiangcuo fault and the restraining bend (Tuosuo- Maqin section) of the East Kunlun fault zone jointly form a wide (~100 km) marginal fault zone of the Bayan Har block. This marginal fault zone, together with the southern Yushu- Ganzi- Xianshuihe fault zone, accommodate eastward motion and deformation of the Bayan Har block in the northern Tibetan Plateau. Since the Maqin- Maqu segment of the eastern East Kunlun fault zone is a seismic gap, future large earthquakes may occur along that segment. In addition, because large earthquakes along the borders of the Bayan Har block appear to have jumped in the last 20 years from one side to another, we infer that the next large earthquake may also occur along its southern boundary: the Ganzi- Xianshuihe fault.