By calculating the daily atmospheric vapor transportation and analyzing the net vapor income over Huaihe river basin during the Meiyu period it was showed that the large scale vapor transportation plays an significant role in enhancing rainfall.It was also showed that there are two major sources of the vapor transportation. One of them is a southwest airflow which originally comes from Bangladesh Bay, and the other is a near south airflow which is converted from the east airflow in the south of the west pacific subtropical high system.It was found that the meridional component of the vapor transportation is always strong and steady during the Meiyu period, while its zonal component had large variations. It was the meridional component that contributed to the net vapor income mainly. The rainfall of this region always became more intense as it increased abruptly and maintained. In the meridional direction, there was a substantial depth layer (mainly from surface to 600 hPa) of vapor transportation that bought vapor in, but the airflow always took the vapor out in the reverse direction. And, the fluctuations of the net vapor income reflected the east Asia Monsoon's movement northward in summer.