Based on the conventional observation data, regional intensive automatic weather station data, GPSPWV data and NCEP 1°×1° reanalysis data, the shorttime heavy rainfall in Nanchang on 21August 2012 is analyzed. The discussion is focused on the causes of the local heavy rainstorm. The results show: The confrontation of a midlatitude trough, the Northeast Subtropical High, and a southeastern tropical system resulted in the accumulation of high energy unstable atmosphere and strong convective movement near Nanchang, and then led to the local rainstorm. The topographic uplift was a direct inducement of the rainstorm. Due to the uplifting effect of the Meiling Mountains, the unstable atmosphere ascended above its free convection height and developed into mesoscale cyclones near the Meiling Mountains. The cyclone moved along the mesoscale convergence line to the vicinity of Nanchang, and maintained for three hours. The rainstorm was composed of a number of strong or extrastrong meso and microscale rainfall centers. The ground mesoscale cyclone, high CAPE values, high and strong water vapor convergence made the MCSs maintained for a long time, which led to mesoscale rainfall centers repeatedly generated and developed around Nanchang. Summarizing the rainstorm flow pattern, we can construct the predictive conceptual model under the condition of weak southwest airflow and provide technical guidance for forecasters to capture such local heavy rainstorms.