Abstract: Based on the correct numerical simulation for a Meiyu front heavy rainfall during 24-25 June 2003 in the area south of the Yangtze River valley, this paper analyzes the mechanism for the rainstorm. The results show that, with the collective influence from an upper-level trough, low-level shear line and upper and low-level jets, warm and cold drafts converge and a Meiyu front forms to cause the rainstorm. During the event, the collective effect of the upper and low level jets is important, and the special velocity field results in three non-divergence layers and two centers of vertical velocity, maintaining deep ascending flows. The 'dry' experiment shows that, the latent heat release heats the deep atmosphere, strengthening the upper pressure and divergence, enforcing the upper-level northwest jet north of the rainfall. Furthermore, the effect of latent heat release makes the lower level decompress and converge, strengthens the lower-level southwest jet south of the rainfall, and maintains the low-level shear line.