A heavy rainfall during 24-25 June 2003 in the Jiangnan area is simulated by the fine meshed WRF mesoscale numerical model developed by NACR, NCEP, FSL/NOAA et al.. Analysis of the output data indicate that WRF model successfully reveals the circulation's evolution and the distribution of rainfall belt. The convergence field non homogeneously located on the low level shear line and the vorticity (positive at middle low level and negative at upper level) contribute to the strong ascending motion. The air is rather unstable near the moisture frontal zone. Results also show the positive centers of local helicity along the shear line match the heavy rainfall centers quite well.