Abstract: Based on the NCEP reanalysis, conventional observation and Global Data Assimilation System (GDAS) data, the characteristics of water vapor transport were analyzed in a historically extreme event of persistent heavy rainstorm affected by a monsoon low pressure in Guangdong province from August 27 to September 1, 2018. Using the Hysplit backward trajectory model, the water vapor source was diagnosed. The results show that:(1) During the continuous heavy rainstorm, the southeast coast of China was the water vapor convergence area in northern hemisphere, and water vapor mainly came from the Indian Ocean, which then flowed northward from the Indian peninsula to the south of the Qinghai-Tibet Plateau, and then turned east into the south of China. Other portion of the water vapor came from the northwestern Pacific Ocean and the South China Sea. The three water vapor sources gathered in the South China and established a stable and continuous water vapor transport channel, which made the torrential rain process wide and lasted for a long time. (2) The water vapor convergence center in the early stage of precipitation occurred on the eastern coast of South China. On August 29th, its center gradually moved westward and the potential function averaged over Guangdong area reached the peak of the whole precipitation event at the night of 29th when the water vapor convergence was the most obvious. At the 31st night, the water vapor convergence center moved further westward and tended to weaken. The change of the high value of the water vapor flux potential function corresponded to the daily variation of precipitation peak during the torrential rain. (3) The daily water vapor convergence showed obvious daily variation. During the daytime, the water vapor convergence was weakened and obviously strengthened at the nighttime, showing the characteristics for monsoon precipitation. (4) The southern boundary of South China was the main water vapor input boundary and mainly concentrated in the lower layer, especially the water vapor input of the southeastern border of South China has been maintained at a high level. The water vapor input from the southern boundary of South China has been significantly increased from the night of 29th, and the whole layer's water vapor flux reached its maximum on the 30th, which was consistent with the area and time of the heavy rain.