Abstract: The paper aims to investigate the mechanism of rainstorm caused by typhoon in late autumn. By using the dataset of CMA- STI tropical cyclone optimal tracks, the NCEP/NCAR reanalysis data, Guangxi regional automatic weather station observation data and precipitation data provided by China meteorological data sharing service, the torrential rain caused by late autumn typhoon‘Haiyan’in Guangxi was analyzed. The results showed that: the water vapor transfer of‘Haiyan’was totally depended on its circulation, but the moisture convergence layer over Guangxi extended to a high altitude. Rains of Guangxi started when the water vapor input of the eastern boundary began to decrease after reaching the maximum, meanwhile the water vapor transferred from output to input and suddenly increased in the southern boundary, and the strongest periods of rainfall occurred before and after the water vapor input of the southern boundary reached a peak. Early invasion of cold air enhanced the baroclinicity of the typhoon and played an active role in the rainstorm, but the warm core structure of the typhoon was completely destroyed by later strengthened cold air and caused a quick packing of typhoon while a fast ending of the rainstorm. The relative storm helicity positive center had good correspondence relationship with heavy rainfall area, and there were at least 6 hours in advance. So it was necessary to put the focus on the water vapor convergence layer and cold air strength during rainstorm forecasting in late autumn, and the relative storm helicity had a good indication in forecasting of torrential rain caused by typhoons.