Based on NCEP／NCAR daily reanalysis data in July 2004, the moisture a nd wind fields of heavy rain events in Guangxi are analyzed. Results show that t he evaluation of heavy rain has a good correspondence to the variety of moisture and watervapor content increased before the heavy rain happened. The occurren ce and break of heavy rain is closely associated with the strengthening and brea k of summer monsoon. There is a remarkable difference of the vapor inflow and ou tflow between heavy rain period and nonheavy rain period. South China Sea is t he main water vapor source, while the Bengal and the West Pacific Ocean also pla y the important roles in heavy rain period besides South China Sea. And the wate r vapor income of heavy rain period is much more than that of nonheavy rain pe riod.