Based on the NCEP/NCAR reanalysis monthly data and daily precipitation data at 20 stations in eastern Southwest China from 1959 to 2006, the features of atmospheric heat source over the Tibetan Plateau and the key area associated with the drought/flood in summer over eastern Southwest China have been analyzed. Results show that there is a close relationship between the flood/drought over eastern Southwest China and the intensity of the atmospheric heat source over southeastern Tibetan Plateau (key area), i.e., the rainfall is more than normal while the heat source is stronger. It is propitious to appearance of circulation making for the summer rainfall in eastern Southwest China; furthermore, the vapor transport and convergent upward velocity are strengthened, the East Asian summer monsoon is weaker, and the ridge positions of the western Pacific subtropical high and South Asia high lean to south when the intensity of the atmospheric heat source over eastern Tibetan Plateau (key area) is stronger than normal, vice versa.