Tongwan city is a focus of Mu Us for discussion. For many years, some experts have tried to prove that Mu Us Sandy Land was once a beautiful place by citing the fact that Haolian Bobo set up his capital in Tongwan city at the beginning of the fifth century. The reason why he gave up Gaoping city where he rose to power and position and established his capital in Tongwan city was that he intended to compete with Northem Wei dynasty in Hedong (East of Yellow River) , at least to keep a balanced position with it, and than conquer Later Qin dynasty in the central part of Shaanxi, and finally unify China. When Haolian Bobo set up his capital there, sand was already in existence. We found fine sand under the ruins of the Palace to the west of Tongwan city when we explored the place in May, 2000. The base of the city wall 13 meters under the ground was built on sand directly, which tallies with the descriptions in book "Shui Jing Zhu" by Li Daoyuan, a famous historical geographer who wrote it after. his inspection tour to Ordos Plateau himself. It proved that there was sand surrounding the Tongwan city, but not desert. The sand has no influence on the decision made by Haolian Bobo to set up a capital city there. Changes in natural environment were contributed to natural forces as well as human activities. The latter had a greater influence on it. A good case in point is that, the scale of both farming and grazing exceeded the land and vegetation carrying capacity because of agricultural and husbandry development, and frequently erupted large scale battles. In addition, the dry weather led to the drying up of lakes. The sand dunes expanded and connected together. Consequently the natural environment took a turn for the worse . Till Tang and Song dynasties, Mu Us had formed sandy environment. After Ming and Qing dynasties, farming scale was further expanded and the areas covered by plants became smaller, which accelerated the formation of desert. Therefore the changes of Tongwan city were resulted from political and military activities as well as natural evolution. We concluded that the city is the reflection of human and social activities in Mu Us area and its rise and fall have little to do with sandy land in Mu Us Sandy Land.