In this study, the long-term trend of water vapor over the Tibetan Plateau (TP) in boreal summer is investigated by using observation and reanalysis data from 1979 to 2019. The historical experiment simulations of 19 models that participated in the Coupled Model Intercomparison Project phase 6 (CMIP6) are evaluated, and the future variation tendency under four emission scenarios is projected. The results indicate that the water vapor content and the net water vapor budget over the TP show notable increasing trends, which are mainly manifested by a significant increase in the net water vapor import and a significant decrease in the water vapor export on the eastern boundary of the TP. This is mainly due to an anomalous anticyclone from Lake Baikal to the Mongolian Plateau. The CMIP6 multi-model ensemble can well simulate the variation characteristics of the TP net water vapor budget. The projection results indicate that by the end of the twenty-first century, the water vapor content, the net water vapor import and precipitation over the TP will increase. Under a high-emissions scenario and compared with the current period (1991–2014), these three variables will increase by 47.99%, 59.77% and 18.59% in the long term (2081–2100), respectively. The significant enhancement of meridional water vapor transport over the northern TP may be the main reason for the increase in humidity over the TP.