Spatial distributions and temporal variations of tropospheric water vapor over the Tibetan Plateau and the surrounding areas are explored by means of the water vapor products from the high resolution Atmospheric Infrared Sounder (AIRS) on-board the Aqua satellite and the NASA Water Vapor Project (NVAP). Because NVAP has serious temporal in-homogeneity issue as found in previous studies, AIRS product is primarily applied here, though similar seasonal variations can be derived in both datasets. Intense horizontal gradient of water vapor content appear along the edges of the Plateau for the lower troposphere (500-700hPa) and the entire troposphere, in particular over the regions along the southeastern boundary. Rich horizontal structures are also seen within the Plateau, but with weaker gradient. In the middle-upper troposphere (300-500 hPa), horizontal gradients are relatively weaker. It is shown that there is always a deep layer of high water vapor content over the Plateau with a peak around 500hPa, which can extend from the surface to roughly 300hPa and even to 100hPa at some locations. This layer of high water vapor has consistent influences on precipitating processes in the downstream regions such as the valleys of Yellow and Yangtze River. Estimated vertically-integrated water vapor flux and moisture divergence in two layers(500-700hPa, 300-500hPa) further confirm the role of the Tibetan Plateau in impacting the downstream regions, which is essential during both warm and cold seasons.