The Xianshuihe fault zone, located in the eastern margin of the Tibetan Plateau, is one of the most active large- scale fault zones in China. Previous studies indicate a great risk of destructive earthquakes along the Selaha- Kangding segment of the Xianshuihe fault in the next few decades. The ongoing, large- scale Sichuan- Tibet railway project is planned to directly cross the several segments of the active SE Xianshuihe fault in the Zheduo Mountain near Kangding. Through interpretation of high- resolution satellite images and detailed field mapping, we discovered a new ~24 km- long, Holocene active fault segment in the Zheduo Mountain granite batholith, located between the Selaha and the Zheduotang fault segments, and named it the ‘South Mugecuo fault’. This discovery of this active fault is of great significance to improve the fine geometric image and structural patterns of the Xianshuihe fault zone, to accurately evaluate its seismic risk, and to scientifically support the construction, operation and maintenance of the Sichuan- Tibet railway.