On October 20, 849 A.D. (October of the third year of Dazhong, Emperor Xuanzong of Tang), a great earthquake occurred near Baotou City, Inner Mongolia Autonomous Region, causing serious destruction of castles, military camps, and civilian houses and thousands of deaths. At present, the basic parameters of this earthquake (e.g., epicenter location, intensity, and magnitude) are controversial. In this work, the relevant historical records were systematically integrated. Further discussion was performed on the seriously affected towns, such as Yunjia Pass or Yunjia Town, because their locations were inconclusive. In addition, the earthquake intensity and locations of the main damaged places were reevaluated, the isoseismal map of the earthquake was modified, and the basic parameters such as earthquake magnitude and epicenter location were corrected. Results show that the isoseismal line of the earthquake in 849 A.D. is asymmetrically spindle- shaped, with its long axis in a nearly east-west direction and its short axis in a nearly north- south direction. The seismic intensity in the meizoseismal area can reach X-X⁺ degrees. The epicenter is located near Baotou City, Inner Mongolia (40.6°N, 110.3°E), with a magnitude of about 7.8±0.3 or 7 . On the south side of the isoseismal line, the earthquake has affected a large area (including Hetao Basin and its south area), with heavy earthquake damage and slow attenuation of seismic intensity. On the north side, almost no earthquake damage has been reported. Correspondingly, the attenuation of seismic intensity is fast, and the affected area is quite small. The isoseismal line shows that this earthquake belongs to a typical normal faulting earthquake, and the seismogenic structure is the middle- western segment of the Daqingshan piedmont fault. Our results on the epicenter location and earthquake magnitude, which are based on historical earthquake data, are consistent with previous findings based on active faults. This study indicates that the earthquake in 849 A.D., which occurred in the northern margin of Ordos, has a magnitude of about 7 . Further study must be conducted on the upper magnitude limit of future earthquakes and the risk of large earthquakes in this area.