Simulations of the Regional Climate Model 3 (RegCM3) under the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A1B scenario were employed to investigate possible decadal changes and long-term trends of annual mean atmospheric water balance components over China in the 21st century with reference to the period of 1981–2000. An evaluation showed that RegCM3 can reasonably reproduce annual evapotranspiration, precipitation, and water vapor transport over China, with a better performance for March–June. It was found that the water vapor exchange between the land surface and atmosphere would be significantly intensified in Northwest China by the mid- to late- 21st century and that the region would possibly shift to a wetter or drought-mitigated state under global warming. Conversely, the water vapor exchange evidently weakened over the Tibetan Plateau and Southwest China by the mid- to late- 21st century. In addition, there appears to be a drier state for Northeast China and the middle and lower reaches of the Yangtze River valley by the mid- to late- 21st century, with slight mitigation by the end compared with the mid-21st century. The westerly and southwesterly water vapor transport over China generally presents an increasing trend, with increasing divergence over the Tibetan Plateau and Northeast China, corresponding to a loss of atmospheric water vapor by water vapor transport.