Abstract: The present oasis of Nan Hu in Dunhuang Xian(County).Gansu province is where the famous Yang Guan(the Sunny Pass ) of the ancient Silk Route was located. It is on this spot that the Lungle Xian had been established in Han Dynasty; then , later ,in Tang Dynasty,it was called Shouchang Xian.The ruins of the city can still be seen with the broken walls half submerged in the aggressing quicksand. Some three kilometrs west of the site of the city ruins, there is the remain of Yang Guan which is also buried by the pushing sand dunes, This sight shows vividly that during the last millennium, that is, since the end of Tang Dynasty, the oasis of Nan Hu had greatly diminished in area. The average annual rainfall of the oasis is less than 30 mm. During the time of Han and Tang Dynasties ,agriculture had been developed in this place. Irrigation system had been built by man utilizing the water source from a natural lake in the southern suburb of the city. Now, the area of the lake is also much diminished. In order to preserve the much needed water for irrigation, a man-made reservoir has been built here. This becomes the artery of the development of agriculture and the survival of the Oasis of Nan Hu. We must consider the fact seriously that the existence of the Oasis of Nan Hu is threatened imminently as the natural vegetation on the upper part of the oasis has been destroyed. The destruction is getting worse and worse in tha past decades with very serious erosion and denudation which cusae the rapid development of deep gorgas on the marginal region of the oasis. The result of such a phenomenon is that in addition to the erosion and denudation, an enormous amount of sand is produced and further the desertification of this area.