Abstract: The features of monsoon activity and its role (Jianghuai) during the 1991 excessively torrential rain over Changjian-Huaihe River valley are analyzed. The results show that the first episodes of torrential rain occurs before the onset of summer monsoon and is mainly induced by synoptic systems in subtropical westerlies; while the activity of southwest monsoon becomes most important during the second and third episodes of torrential rain when subtropical high in West Pacific and southeast monsoon stay stably. The maintenance and break of Meiyu are as a result of the activity of southwest monsoon. A significant effect of southwest monsoon is to transport convective air from tropical area (Arabian Sea and the Bay of Bangal) to Jianghuai valley, thus the convective rain formed in the middle and late stages of the Meiyu is much stronger than that in the normal year, especially in the third episode. The distribution and propagation of strong wind core along southwest airflow are also studied