Abstract: The reason of the site selection of Tongwan City, which located on the southern edge of the Mu Us Sandy Land, is always the research focus. History scholars regard it as space-time coordinates of historical ecology, culture, policy and economy because of its grand scale and important position. As the capital of Daxia, its surviving time was only from AD 418 to AD 427, just 9 years. It is called "its rise for Helian Bobo and its fall for negligence ".This paper consider that the selection of the city as the capital of Daxia is unreasonable to some extent based on background of physical environment, resources and military defense. Helian Bobo was brave but he did not have a comprehensive plan of the Tongwan City as the capital of Daxia. The Tongwan City could not meet requirements of physical environment, resources and military defense as a capital city. As a result, when the city built, the military of north-wei country attacked and overcame it soon, and the Daxia perished. Later,the administrative position of the city gradually reduced, eventually buried by the sand.

Key words: Tongwan City, capital of Daxia, physical environment, resources carrying capacity, military defense, Mu Us Sandy Land