Abstract：The Xar Moron River Fault is the final suture between the North China Plate and the Siberia Plate. The location of its western part has been well known, but the extension of its eastern part remains unclear and has aroused much controversy, especially concerning its location under the Songliao Basin. Based on field investigation along the western margin of the Songliao Basin (Nenjiang-Balihan fault) and previous researches on the Central Fault in the Songliao Basin, it is found that the Nenjiang-Balihan Fault and the Central Fault were strike-slip faults at the early stage. They displaced sinistrally the Xar Moron River Fault and made it extending in a staircase form west to east. Based on the available high precision age data from the basement of the Songliao Basin, the reinterpretation of the gravity anomaly and MT profile, the characteristics of the lithosphere and crustal thickness of the northeast margin and the neighboring areas of the North China Plate, the authors revealed the distribution of the Xar Moron River Fault under the southern Songliao Basin, which extends from Kailu, Tongliao, Horqin Zhuoyizhongqi to Changchun.