Based on the precipitation data at regional automatic stations and conventional weather stations of Hunan Province, NCEP/NCAR reanalysis data, JRA-55 reanalysis data and the forecast products of atmospheric river (AR) from Hunan Provincial Meteorological Observatory, the characteristics of heavy rainfall and abnormal water vapor transport of the flood-causing torrential rainfall in Hunan Province from 22 June to 2 July 2017 and the influence of AR water vapor transport on heavy rainfall were analyzed. And on this basis the water vapor budgets at each boundary over heavy rainfall area and the contributions of water vapor trajectories were analyzed quantitatively. The results show that the torrential rainfall process had three stages, the range and intensity of rainfall at the first and the third stages were significantly greater than those at the second stage. The stable circulation situation with ‘one trough and one ridge’ in middle and high latitudes of Asia and Europe, the relatively stable subtropical high in low latitude and its peripheral strong water vapor transport were weather background of the heavy rainfall. The horizontal and vertical distribution of physical quantities such as water vapor flux, water vapor flux divergence and specific humidity had great indication to the periodic characteristics and the change of location and intensity of rainfall. The intensity and location of water vapor transport from the Bay of Bengal, the South China Sea and the southwest side of the subtropical high were different at three stages of the heavy rainfall process, which caused the spatial difference of southerly abnormal water vapor transport to Hunan. The intensity of AR and its water vapor transport channels, the location of convergence zone and the net water vapor income at each boundary over heavy rainfall zone played a key role in the occurrence and development of the heavy rainfall. The southerly water vapor transport in low level was an important factor of the heavy rainfall lasting for a long time, while the invasion of dry and cold air from the north was beneficial to the enhancement of atmospheric baroclinicity and the maintenance of convection instability, which was another reason why the intensity of rainfall at the second stage was weaker than that at the first and third stages.