The Meiyu front rainstorm occurred in the period from 28th to 29th July,1998 in the Yangtze River's middle and lower reaches is dynamically diagnosed and numerically simulated using the MM5,and results indicate that the temperature and moisture disturbance in the lower level plays an important role in the development of convection and the generation of the rainstorm.The joint occurrence of low-level jet and wind shear is beneficial to the out-growth of the strong rainstorm.The structure of moist potential vorticity field in the rainstorm period differs greatly from those before and after the period.