Earthen sites, such as the Great Wall of China, are important elements of cultural heritage, but are at high risk of erosion due to environmental changes. In this study, unmanned aerial vehicle low-altitude oblique photography was used to assess the erosion of the Ming Great Wall in Gansu Province. The erosion characteristics (height, depth, area, and ratio) were quantified using a 3D point-cloud model. Combined with onsite sampling and analysis, the deterioration distribution was examined, and the progression of damage summarised using historical images. The degree of erosion in the rammed earth Great Wall was linked to the soluble salt content in the soil. The degree of deterioration of the walls indicates a significantly larger hollowing area on the southern side than on the northern side, and a slightly larger area on the western side than on the eastern side. This paper addresses the challenges of assessing and quantifying erosion development in specific segments and provides a risk assessment of erosion at any point in each segment. It also provides a valuable reference and scientific support for the protection and restoration projects of the Great Wall during the Ming period.