Based on the synoptic theory, a heavy rain event in East China during 10—12, Oct. 2003 is studied. Results show that the monsoon cloud system generated in the Bay of Bengal and the spreading northeastward monsoon cluster and the upper-level jet cloud are the main synoptic systems. The interaction of low-level jet and upper-level jet attributes the good dynamic condition. After the analysis of the cloud images data and numerical products, it is found that the heavy precipitation is located in the area with characters both low-level strong convergence and strong vapor supplement.