Abstract: A heavy rainfall in the meiyu front during July 4-5, 2003 is simulated by use of the nonhydrostatic meso-scale model MM5 (V3.6) with different explicit cloud microphysical parameterization schemes. The characteristics of microphysical processes of convective cloud are studied based on the model outputs. The simulation study reveals that: (1) the mesoscale model MM5 is capable of simulating the instant heavy rainfall in the meiyu front. The rainfall simulation is significantly improved when the model resolution is increased. And the Goddard scheme is better than the Reisner or Schultz scheme. (2) The convective cloud in meiyu front has a comprehensive structure composing of solid, liquid and vapor phases of water, the mass density of water vapor is the largest one in the cloud; the next one is graupel, while those of ice, snow, rain water, and cloud water are almost equal. The mass density peak heights for different coagulations are almost unchangeable during the heavy rainfall period. The mass density variations of rainwater, cloud ice and graupel are consistent with that of ground precipitation, while that of water vapor in the low levels is 1-2 hours earlier than ground precipitation. (3) The main contribution to the water vapor budget in the atmosphere is the convergence of vapor flux through advection and convection, which provides the main vapor source of the precipitation. Except the basic process of the conversion of cloud water into rainwater, there is an additional cloud micro-physical process that is dynamically indispensable for the formation of instant heavy rainfall. In this process, the icephase crystals between 500-700 hPa converse into graupels first and then the enhanced mixing of graupels and cloud water accelerates the conversion of cloud water into rainwater. The positive feedback mechanism between latent heat release and convection is the most important thermal factor promoting the maintenance and development of the heavy rain.