By means of the intensive observation data of the second TIPMEX(Tibetan Plateau Meteorological Science Experiment) in 1998 and the NCEP/NCAR reanalysis data,the diagnostic analyses on water vapor fluxs during the key period(June to July) of heavy rainfall processes in 1998 are performed,and the water vapor inflow on the lateral boundaries is numerically inrestigeted in this paper.The results show that the exceptional torrential rainfall processes of 1998 in the Yangtze Basin occurred under favorable conditions of sustained anomalous and steady synoptic scale environment and meso-scale wind fields;the water vapor transport feature in June was different from that in July;the water vapor transport on both the west and south boundaries of the region played an important role in the torrential rainfall processes,i.e. The noticeable change in the water vapor source and its corridor was important to the formation,and development of torrential rain at different stage of the meiyu period in the Yangtze Basin;the spatially and temporally distribution characteristics of water vapor gave some important clues for the prediction of sustained exceptional torrential rain over Yangtze River basin.