Abstract: Based on the NCEP/NCAR monthly reanalysis data from 1979 to 2008, this paper has analyzed the variations of the atmospheric water resource over the arid region of Northwest China (ARNC) from the aspects of water vapor transport, precipitable water vapor and water vapor budget, and then has revealed the main causes of the variability. The results show that the annual precipitable water vapor over ARNC has shown increasing trend during the recent 30 years, while the summer water vapor net income has displayed an even more remarkable increasing trend. The westerly water vapor transport in the middle latitude had very important contribution to the variability of the atmospheric water resource over ARNC. The interdecadal descending characters of the regional westerly wind index implies the weakening of the water vapor transport both at the eastern and western boundary of ARNC, the descending trend of the water vapor budget at the eastern boundary, however, was much more significant than that at the western boundary due to the effects of the huge geomorphological structure of ARNC, thereby resulting in a lot of water vapor stranded in the arid region, and the total summer water vapor budget increased.