By using conventional meteorological observation data, NCEP reanalysis data, satellite and radar data, densified surface observational data, etc., a rare rainstorm process which happened during June 17-20, 2010 in northern Jiangxi was analyzed. In this paper, the heavy rain process was analyzed from three aspects: dynamics and diagnosis analysis, mesoscale analysis, and simulation analysis by WRF model. The analysis results show that: (1) This rare rainstorm is a typical heavy rain over Meiyu front, it is caused by the vigorous development of strong mesoβ system generated in the extremely favorable weather situations. The cold air flow behind the 500 hPa East Asia trough joined up the SW flow located in the northwest part of the strong and stable subtropical high, thus the cold air and warm air converged and maintained over the northern part of Jiangnan.(2) Since the area that cold air and warm air joined up is stable and the SW warm and wet flow is abnormal strong, the vapor, dynamical, and thermodynamic conditions leading to heavy rain are quite adequate, the physical quantities are significantly higher, these are very helpful to trigger the strongly development of meso and microscale convection systems.(3) The extraordinary rainstorm is caused by the interaction of many factors such as strong vapor and convergence ascending motion, weak cold air activities in middlelevels, the strengthening of SW lowlevel jet, the weakening of stablility of convection layer, stability of surface convergence lines, the formation and maintenance of mesoβ vortexes and strong divergence in higherlevels, etc.(4) Numerical simulation analysis of WRF model shows that at 08:00 BT 19 June 2010, the formation of mesoβ vortex is related to the strengthening of weak cold air and warm and wet flow, the vertical thickness of this vortex is between 950 and 550 hPa, and it is the strongest between 800 and 950 hPa. In addition, a string of intense convective systems appear in the southern part of this mesoβ vortex, these systems are nearly in eastwest direction and the scales are about 30 to 60 km, and their positions are well corresponding to the severe rain area.