Based on the NCEP/NCAR daily reanalysis data,the circulation background of the flash-flood-producing rainstorm over South China in June 2005 and its possible cause are analyzed.The results show that the weak ElNino events during 2004-2005 was the climatic background of the rainstorm.Compared with summer in 2005,the apparent heating source <Q₁>and apparent moisture sink <Q₂>were exceptionally strong over South China during June 17-26,2005.And the center of high value bands of <Q₁>and <Q₂>were basically coincided with that of rainfall.The anomalous heating source over the east coast of Arabian Sea and the Bay of Bengal forced an anomalous anticyclonic circulation over its northwest at the upper level,leading to the strenthening and westward stretching of South Asian High(SAH).An anomalous cyclonic circulation controlled the Chinese mainland,which went against the northward movement of SAH,but favored the maintaining of SAH over the south of the Tibetan Plateau and South China,so South China areas just located in the updraft areas in the south side of the upper level westeries jet,which is frequently in favor of the formation and maintenance of heavy rain,and thus severe flood.