An analysis of the convection cloud cluster variation over the Bengal Bay and the features of circulation at both high- and low-level atmosphere during heavy rain processes in Guangxi occurred from May to July, 1980 to 2002 is made. The results shows that convective cloud clusters on the Bengal Bay develop most strongly when there occurs heavy rainfall over Guangxi three days ago. Leading by the 500 hPa Bengal Bay though, the Bengal Bay convective cloud clusters climb up the Indo-China Peninsula and come into Guangxi. When the Bengal Bay convective cloud clusters join the convective cloud clusters moved from the Tibetan Plateau, the clusters develop strongly again, then resulting in heavy rainfall around Guangxi. Analysis of the height and wind flow pattern at 200 hPa shows that the Bengal Bay, the Indo-China peninsula, and Guangxi are under the control of the SouthAsia High during heavy rainfall over Guangxi. Analysis of the distribution of the vapor flux vectors at 850 hPa shows that there is a water vapor transmitting belt from the Bengal Bay to Guangxi, and the Bengal Bay is the vapor sources of heavy rainfall over Guangxi.