In 2005, when studying the process of historic sandy desertification on Erdos Plateau, we found that a lot of ancient city ramparts were fortified by alternate layers of sand and soil, and these cities included Shi'er Lian Ancient City (located in Zungar Banner, Inner Mongolia, China), Dabaodang Ancient City (located in Shenmu county, Shaanxi province, China), Bayanhurihu Ancient City (located in Otog Front Banner, Inner Mongolia, China) and so on. They are located in deep desert now. Several researchers have thought that the fate of these ancient cities is associated with natural environmental conditions. However, our experimental result does not support this view. The discovery sheds new light into when and how sandy desertification occurred and provides new historic insight for eco-environment conservation in this district. Through the experiment, some conclusions are drawn as follows: (1) The sandy layer in the ramparts consisting of aeolian-sand which should deposit before the cities were built. Considering the time when these cities were built, we could conclude that the desert scene had existed before the Han Dynasty. Some researchers pointed out that the existence of these cities were associated with good natural conditions when they were built, while our experimental result did not support this view. Accordingly, the existence of these cities could not be an evidence to prove good environmental conditions on Erdos Plateau in historical times. (2) It is an effective way to analyze the environmental change and process of man-land relationship through the fate of the ancient cities, relics, traces of human activities and changes in wasteland reclamation. However, it is not accurate to quantify the time when sandy desertification occurred through the time when these relics were abandoned. Undoubtedly, farming activities in historical times to a certain extent destroyed vegetation on Erdos Plateau, thereby accelerating the process of sandy desertification, but it is not the main factor leading to sandy desertification before the Ming Dynasty. (3) It is credible that sandy layer in the ramparts as a direct sign to interpret the environmental background, so it is very useful for us to explain the process of sandy desertification in historical times. Through the discovery, mobile sand dunes, fixed sand dunes, meadows in low humid lands, lakes, and so on, can coexist on Erdos Plateau in prehistory and human history. Up to now, these environments still coexist in the district.