In order to figure out the natural environment during the construction period (around 1600 years ago) of Tongwan city and a long-standing controversy of whether the city is built on sand deposits, research is done on six profiles of sample bore holes by grain size analysis, X-Ray diffraction, etc. This study indicates that lacustrine deposits are identified under the surface of ground both inside and outside of the city wall, and their features are totally different from wind deposit. The differences are shown as follows: (1) There is yellowish-brown goethite in targeted sand layers. (2) Several dark gray deposited layers are identified. (3) The content of coarse sand in the targeted sand layers is much higher than that in wind deposits. (4) The grain content showed a mixture of coarse and fine sand with a higher degree of separation. (5) The contents of Fe2O3 and A12O3 in fine-sand spots in lacustrine deposits are higher than those in sand deposits. Supported by concrete evidences collected, it is shown that the natural environment of the city was good during the construction period. And the basic landscape is plateau generated owing to the accumulation of lacustrine deposits, on which the ancient city was built. Desertification did not occur during the construction period, but did during a period after its construction.