The destruction of plants root, especially trees and shrubs, on geotechnical ancient sites is a concerned hotspot by conservators. The primary task for the weathering and mechanical damaging mechanism study of earthen ruin is how to monitor the spatial distribution and diameter sizes of plant roots around ancient sites. In this study, we aimed the Beiting Ancient Town earthen ruins, using the ground penetrating radar technology, to study the spatial distribution of elm roots that extend to the ancient city wall, and the possible influence of belowground roots on earthen city wall ruins has been deduced. The results indicated that dielectric constant ranged between 8.73 to 19.22 in four detected sites, mainly related to the soil water content and medium density belowground. Meanwhile, the roots diameter ranged between 50 millimeter to 400 mm, the minimum resolution was 50 millimeter, most of roots diameter distributed between 100 mm to 250 mm, the tree-age is the key factor that determine the roots diameter and distribution traits. Considering the tree-age, percentage of roots diameter sizes, extension direction of big roots, and relationship between trunk and ancient sites position, we confirmed that the roots with the most threaten degree to ancient city wall were, in order, the third site, the forth site, the first site, and the second site respectively. The nondestructive ground penetrating radar technology can monitor roots that grow in or around historical remains, which conform to the basic principle for cultural heritage conservation, and has very good application value in the scientific evaluation of biotic factors as roots influences on ancient sites.