Distributive characters of water vapor in Western China are analyzed in this paper by using the NCEP monthly mean specific humidity data from 1958 to 1997.Analysis results show that the structure of the vertical distributions of water vapor is very similar,the center of water vapor above the 850hPa level laid over the Qinghai-Xizang Plateau,and the water vapor content mainly concentrated in the lower atmosphere below the 500hPa level in May to October in a year,especially in July moisture was most abundant.The content of water vapor reduced obviously with altitude.It was largest in summer,next in autumn,and smallest in winter.The interdecadal variations of moisture in the recent 40 years show that summer moisture has linearly reduced,especially since the 1990s;while winter specific humidity has linearly increased;and the interdecadal trends of January and July specific humidity were oppsite in phase.