Having extremely high artistic, scientific and social values, earthen sites are widely distributed in China, which are important cultural relic resources of mankind. Due to accumulated natural erosions and human activities, however, earthen sites have been greatly damaged and have even vanished many sections, so that they are seriously threatened by extinction. In the circumstances, acquiring the conservation status of earthen sites is a vital prerequisite for the subsequent targeted protection. In this paper, as a world-renowned heritage site, the castles of the Ming Great Wall located in Qinghai Province were selected as the research object. A novel indicator namely the linear vanishing section ratio (L-VSR) was proposed and its value was classified as five levels to quantitatively characterize the specific conservation status of such sites. Based on the assessment results, the castles at the excellent status constituted the minimum proportion, while the castles at the very poor status represented the largest percentage, reflecting the grave situation of earthen sites. Furthermore, applying population distribution models into the linear fitting combined with the population density (PD), the positive correlation between PD and L-VSR was obtained to reveal the anthropogenic influence on the destruction of earthen sites. This study provides a new thought to quantitatively characterize the preservation conditions of earthen sites and also indicates the effect of human activities on the damage of earthen sites from the population perspective. Hence, it is beneficial to the overall damage assessment of earthen sites, and also meaningful to their further preservation and protective planning.