Abstract: A severe tropical storm develops over the Bengal Bay at the mid May, 2004. Jointly affected by the Bangal Bay storm and cold air and wind shear, a heavy rainstorm which lasts 3 days hits Yunan Province with the rainband stretching mainly from the west to the south. From 08:00 (BT) May 18 to 08:00 (BT) May 19, a severe rainfall occurs in west Yunnan with the centers in Baoshan, Dehong and Lincang. 24-hour rainfall is over 100 mm in 6 county stations, the rainfall center is at Longling of Baoshan with the 24-hour rainfall reaching 124.3 mm. It is the first time that such a concentrated heavy rain occurrs in a circular area of 180 km radius in Yunan.The tropical storm is one of the important weather systems which cause severe rainstorm in China. There are a lot of scientific investigations on the occurrence and evolution of the tropical storm (typhoon) over the west Pacific Ocean. Thus the forecasting ability on the tropical storm (typhoon) over the west Pacific Ocean has been improved. However, researches on the tropical storms originated in the Bengal Bay are still few. In fact, the storm of the Bengal Bay has significant impacts on the rainstorm in the southwest China. To explore the physical mechanism of the heavy rain in west Yunnan occurring on May 18, 2004, the MICAPS products, FY-2 satellite images and Dehong CINRAD-CC Doppler radar volume scan data are taken into account for the investigations. Moisture flux divergence, K index, S_i index and Q vector divergence show that the synoptic scale affecting systems are the storm over the Bengal Bay and the southward cold front shear. The heavy rain occurs in the favorable environment field where the centers of water vapor convergence with high instability energy and high wet, positive helicity region at 700 hPa level and convergence area of wet-Q vector are located. Also, the satellite images present that the evolutions of some meso-β scale convective systems are the key of the heavy storm. And Doppler tracking observations give the facts that the echoes are mixed wadding rain echo with intensity between 30~44 dBz, the direct factors of the process are the meso-and small-scale systems such as frequent inverse wind, low level jet and meso-scale convergence line.