Abstract: Based on the data of regional automatic weather observation station,the NCEP 1°×1° reanalysis fnl data and CINRAD-CC Doppler weather radar data,a rare rainstorm is analyzed occurring in the northwest of the Hunan from 24 to 25 September 2013.The results indicate that it developed in the background of Siberian trough dipping forward and subtropical high keeping stable in the southeast of china.Two axis of jet stream were formed because of Typhoon and the northward and northeast direction water vapor channel set up.A large amount of warm-moist airflow was accumulated in the windward slope of the Wuling Mountains over the Nanling.Moreover,the special topographic features 1 of Wuling Mountains (northeast to southwest) and Youshui River (which goes from the north and south after to the east and west) contribute to the collection of the northwestern cold air.Intersection of the cold-warm air was the trigger mechanism of this rainstorm. There was train effect at radar images.The high value area of evolution of Vertical velocity,wind,water vapor flux,the divergence of water vapor flux and pseudo-potention equivalent temperature at 850 hPa correspond well with the falling area of the heavy rainstorm.