Numerical simulation is made on the heavy rainfall over the Huaihe valley on 3 to 4 July 2003 using the PSU/NCAR MM5 model. It is revealed that the MM5 model performed quite well in simulating and can provide highresolution data for diagnosis. The mesoαscale and mesoβscale convective systems of about 100 km can be simulated out by the MM5 model, but the model has a limited capability in simulating the mesoβ(γ)scale systems of smaller scales or more strongly developed systems. The direct reason of the heavy rainfall event is the rapid development of the about 100 km long mesoβscale convergence zone at the lower levels, which is validated by the highresolution data provided by the MM5 model. The mechanisms and the 3D structure of the mesoscale convergence zone are obtained by the diagnostic analysis of the physical quantity field characteristics.