The spatial and temporal characteristics of water vapor from 500 hPa to 200 hPa level in winter and summer are analyzed by using the monthly mean radiosonde data at various levels of 16 stations over the mid eastern Tibet Plateau from 1979 to 2008 The results indicate: (1) The spatial distribution of winter and summer water vapor over the mid eastern Tibet Plateau shows obvious northwest southeast direction. The inter annual fluctuation of vapor over the southern plateau is bigger in winter and summer, relatively stable in the northern plateau. Water vapor changes identically in general in summer and winter, and there also exists north to south reverse phase regional anomaly change in summer, while there is east to west reverse phase change in winter. (2) The water vapor in summer shows a weak rising trend over time generally, negative during 1979-1995 and positive during 1996-2005, with the breakpoints in 1997 and 2006; the water vapor in winter as a whole shows a slightly decreasing trend, negative during 1979-1984, stably during 1985-2004 with the breakpoints in 1985 and 2005 In general, there is a positive trend in vapor over the western plateau and a negative trend over the eastern plateau.