Abstract: Based on its definition,the relative raining index of 91 stations in Asian monsoon region was calculated with monthly mean rain data from 1961 to 1990.The mark of rainy season onset was defined as the index exceeding 147.According to this standard,the area whose rainy season began first was located in the north part of longitudinal range-gorge region in southeast of Tibetan Plateau.Rainy season in this area,known as peach blossom flood,started in March.The elementary reason for peach blossom flood was also studied.Affected by the terrain of the plateau,wet southeast flow rose along the longitudinal range-gorge region toward the tableland in low latitude and the cold advection came with north flow from Tibet subsidized along the longitudinal gorge region.The combination of these two flows induced the onset of rainy season in this area.