Under the guidance of eco-city theory, and its practice in China, we use grid calculation method of GIS & landsat pictures of remote sensing to assess the urban eco-environment quality, e.g. the situation and quality of water, air, land & soil in Longxi County, which is a poverty stricken county in long history in west China. Then we use the dynamic system to simulate the urban system of Longxi County, in which we contrast two kinds of developing models, i.e. the normal & transitional ways, and three types of indexes to evaluate the benefits of the developing model, i.e. the environmental, social and economical indexes. We try to find out the driving forces of the transition, which are population, economy, technology, governance, etc. The change of driving forces is the main factor that shapes the future Longxi, and the model shows that if the local policy makers had adopted the eco-city theory, the benefit of transitional eco-city would exceed the normal developing way, and the urban ecology would also be more stable, and the economy of Longxi would be more efficient. So only if the Longxi County were an eco-city, which composed by many eco-towns, eco-villages (some villages have begin to put this theory into practice), that the urban development of Longxi can be sustainable, compared with the normal developing way. And the main economic activities in Longxi will also be the recycling-economy, and resources-saving society, which can best utilize the resources of current Longxi.