Abstract: In order to improve medium range prediction of torrential rain in Guangdong Province, the variation characteristics of the torrential rain in first rainy season and their relationships with biweekly oscillation of a key region at 500 hPa geopotential height are investigated with wavelet analysis, power spectrum analysis, cross spectrum analysis and Lanczos filter using the daily precipitation data of 86 stations and rainfall regimes, and NCEP/NCAR daily data for the last 48 years of 1961-2008. The results show that torrential rain during June has increased in both the number of days and intensity since the 1990s, but the total number of torrential rain days for the season has decreased since the end of the 1990s, exhibiting significant quasi periodic oscillations of 6—7 years. The storm rainfall accounts for 37.7% of total rainfall during Guangdong first rainy season, and is significantly positive correlated to the total rainfall. The area (20°-30°N, 102.5°-120°E) with 500 hPa geopotential height is significantly negative correlated with the daily precipitation during April-June in Guangdong, named as the 500 hPa key region. The daily precipitation during April-June in Guangdong and 500 hPa key region exhibit significant quasi weekly (5—9 days) and quasi bi weekly (10-20 days) oscillations but less significant 30—60 days oscillations. The relation between 500 hPa key region and the daily precipitation during April—June in Guangdong Province in the quasi biweekly oscillation scale is most close connected, and the lead or lag time of oscillation is within 2 days. From April to June over the 48 years, the probability of torrential rain is 79% within three or four days (in fewer cases) before or after the valleys of the quasi biweekly oscillations in the 500 hPa key region. The characteristics of atmospheric circulation with or without torrential rain in Guangdong Province for quasi biweekly oscillatory valleys of 500 hPa key region are analyzed with the composite analysis of typical examples. When the torrential rain happens near the valleys, the cold air moves southward by significantly enhanced trough over East Asian, meanwhile, trough over the Bay of Bengal significantly deepens and widens, and torrential rain in Guangdong Province are caused by the interaction between north cold air and southwest warm and moist air brought by strong southwest flow from the Bay of Bengal. When no torrential rain happens near the valleys, the cold air acts northward, no significantly transfer of water vapor moisture transport from the Bay of Bengal, Guangdong Province is controlled by strong southwesterly at the verge of subtropical high and is located in the divergent region of moisture flux, the rainfall is weaker. All these are not conducive to the occurrence of rainfall, especially torrential rain. Therefore, the significant difference between the circulation fields can be used as reference for medium range forecast of torrential rain in Guangdong Province.