Abstract: For purpose of understanding the causes of the torrential rain with flood hazards occurred in Nanchang area on June 22 2015, the conditions and possible causes of mesoscale convection weather influence are analyzed and discussed by analyzing data from conventional observations, Fuzhou Doppler radar, FY-2F meteorological satellite imagery and NCEP reanalysis. The result shows that this torrential rain event occurred in the region of northwest subtropical high, with vertically-uniform southwest airflow right before a trough. This disastrous rain event with evident convection was formed and maintained in a warmer region. The possible causes include the combined effects of the cold trough and subtropical high, abnormal abundant moisture, superimposed influence of gradually increased advection of warm and cold air in middle level and high unstable energy. Significant warm humidification process makes convection conditions near Nanchang area more favorable than its surroundings. The lifting condition was also enhanced by combined effects of mountainous terrain in west and the Poyang Lake water, of which the combined effect resulted in the enhancement and maintenance of the regional torrential rain with disaster. When the mesoscale convective cloud was in growing stage, heavy rainfall of above 30 mm·h^-1 mostly happened rear to the part of cold cloud cover of TBB lower than -62 ℃. The presence of easterly airstream and its convergence in the boundary layer intensified the development of precipitation echoes. The "train effect" caused the continually heavy rain, and the time variation of echoes' VIL values show a good indication for forecasting the short-term rain intensity.