Abstract: Since the current monsoon formed in mid-Pleistocene, a vast transitional zone from the sem-i arid area to arid area, i. e. a farming-pastoral ecotone, has formed from Hulunbeir on the western slope or Daxinganling in the north, and extended southwestward through southeast Mongolia, north Heibei and north Shanxi to north Shaanxi and Erduosi Prairie in north China. Its agricultural and pastoral areas lay in spatial distribution and shift in temporal sequence. Located in a vulnerable ecological environment zone, the farming-pastoral ecotone in north China is charaterized by the serious ecological environment vulnerability, poor carrying capacity, obvious degeneration and frequent occurrence of natural disasters. Based on the synthetic analysis on the ecological environment in the farming-pastoral ecotone, in this paper, the eco-environmental vulnerable mechanisms of land desertification are discussed. The results are as follows： (1) The peculiar geomorphic pattern and the surface substance base for desertification are steeled by the vulnerable natural conditions, and the land desertification in the farming-pastoral ecotone is driven by the climatic conditions which is influenced by east Asian monsoon; (2) The irrational human activities are the direct motive forces for the land desertification; (3) The process of land desertification is sped up by the disadvantageous social factors. Land dertification is a synthetic effect of the vulnerable natural conditions and the irrational economic and social factors on the vulnerable ecological environment.

Key words: vulnerability of ecological environment, land desertification, farming-pastoral ecotone, China