A heavy rainfall event in the lower valley of Changjiang river on 25 June 2004 is simulated by the mesoscale numerical model MM5 and is analyzed with the synoptic charts and satellite imagine. The results indicate that under the favorable weather background, it is caused by the strengthening again of the southwest low level jet and the movement of the low level shear line. The analysis of helicity also revealed that under the updraft, the conversion from the horizontal whirlpool into the vertical whirlpool is one of the possible mechanisms of the event.