Analysis of sudden heavy rain in the middle and south of Fujian Province on Sept. 20, 2003 is made. Results show that Typhoon Choi-wan, turning to north far away, plays an important role of organizer and promoter in the formation of the heavy rain. Through the adjustment of the general circulation, the typhoon makes an orographic depression and a westerly trough develop. As a result, the cold air invades southwards more intensively, which is a key trigger for the heavy rain. The outflows downward at the west side of the typhoon enhanced the low level inflows of a mesoscale vortex and constituted a secondary circulation together with the updrafts over the heavy rain area, which strengthen the development of the convective cloud clusters.