Abstract: An index to define the days of torrential rain in Uuangdong was advanced to facilitate automatic judgement by computcr using 51-year (1961-2011)86-station daily precipitation observations of Uuangdong Province.The variation characteristics of sustained torrential rain during the pre-flooding season and its each month in Uuangdong were analyzed.The circulation pattern and the water vapor transport were diagnosed by using daily atmospheric data from NCEP-DOE Rcanalysis 2 during 1979-2011.It was shown that the present index of torrential rain in Guangdong may avoid unreasonable situations of the originali ndex and manual operation.And the interannual variations of both the number of days and the amount of cumulative rain fall of torrential rain derived from the present index can reflect better the drought and flood situation of Uuangdong.The percent of sustained torrential rain is 10%,29% and 61% in April,May and June respectively averaged over the 51-year.Most of sustained torrential rain occurs in May from latter stage of 1960s to 1980x.But most of sustained torrential rain occurs in June during early and middle stage of 1960x, and during 1990-2011.The inter-decadal variation is resulted from the change of atmospheric circulation systems in the middle and low latitudes of East Asia.In addition to the two types of circulation pattern used to be known as "3-ridge and 2-trough" and "2-ridge and 1-trough",another type of circulation pattern in the middle and high latitudes at 500 hPa is found,which is "blocking in high latitudes-placid in middle latitudes" (named as pattern Ⅲ).Pattern Ⅲ occurs in Junc, account for 22% of the total sustained torrential rain processes in June.So the reveal of the circulation pattern Ⅲ would decrease the forecasting omission of sustained torrential rain process in June.The intensity and form of troughs (or ridges) over each region change with months because of the changing of climate background among the different months,for either the pattern of "3-ridge and 2-trough" or the pattern of "2-ridge and 1-trough".For example, for the pattern of "2-ridge and 1-trough",the high ridge over northeastern Asia intensifies and expends northward,the trough over Lakc Baikal widens, the trough cast of Arabian Sca in the middle and low latitudes moves westward,and the trough over Bay of Bengal and the western Pacific subtropical high strengthen month by month from April to June.The primary origin of water vapor ter vapor flux are the necessary conditions for the sustained torrential rams.The sustained torrential rain is resulted from the for the sustained torrential rains during the pre-flooding season in Uuangdong changes with month.The water vapor for the sustained torrential rains comes mostly from the tropical western Pacific in April,from both of the tropical western Pacific-the South China Sca and Bay of Bengal in May, and from Bay of Bengal in June.For all kinds of sustained torrential rain,most parts of Uuangdong arc the convergent area of water vapor flux.This demonstrates that both the dynamic convergence and water vapor flux are the necessary conditions for the sustained torrential rams.The sustained torrential rain is resulted from the different types of circulation pattern in the middle and high latitudes together with water vapor transport from the tropics.