A mesoscale analysis for the flash flood process in early summer of 2005 in Hunan Province is performed with the surface densified observation, rainfall data of automatic precipitation stations, FY-2C satellite TBB, Doppler-Radar data etc. The results indicate that the heavy rain occurred under the effect of mesoscale convergence line, mesoscale low and meso-α convective cloud clusters together. Suction effect with the divergence in the high level emerging earlier than that of convergence in the mid- and low-level, unstable stratification of the dry and cold air in the high and warm and humid air in the low in accompany with the bow pattern of θ_se profile and rich water vapor transport from South China Sea are the environmental conditions for the occurrence of heavy rain. Mesoscale systems which are corresponding with the activity of rain clusters can be clearly seen from the stream field after filtering. The development of cumulus and convection is earlier than that of mesoscale systems and rain clusters. Heavy rain is closely connected with the deep convective cloud clusters and convective cells with the strong radar reflectivity, high echo top and big VIL.